Identification of a second active site in laminin for promotion of cell adhesion and migration and inhibition of in vivo melanoma lung colonization

Previously we reported that a pentapeptide (Tyr-Ile-Gly-Ser-Arg or YIGSR) from domain III of the B1 chain of laminin is a cell attachment site with the ability to stimulate cell adhesion and migration and to block experimental metastases. Here we report studies on the activities of synthetic peptides that cover domain III and report a second biologically active peptide PDSGR from this domain with activities similar to YIGSR. We also show that cyclic YIGSR is more potent in these assays than the linear peptide as expected since this sequence on laminin is bracketed by cysteines. Due to their proximity and similar spectrum of activities, it is possible that these sequences act in concert in the native laminin molecule.     

Cell adhesion, spreading and neurite stimulation by laminin fragment E8 depends on maintenance of secondary and tertiary structure in its rod and globular domain

The cell adhesion, spreading and neurite-promoting properties of mouse tumor laminin fragment E8, which contains major site(s) responsible for laminin-cell interactions, were probed by proteolytic degradation, denaturation, synthetic peptides and antibody inhibition. Removal of more than half of the N-terminal portion contributing to the rod-like domain did not effect cell attachment or spreading although neurite-promoting activity was reduced. More extensive degradation of the rod or of the globular domains of E8, or separation of the globule from the rod, also resulted in loss of cell spreading activity although weak attachment was found to an A chain subfragment comprising the globular domain and a short piece of the rod. Exposure of E8 to increasing concentrations of dissociating agents produce an apparently reversible denaturation but an irreversible loss of both attachment and neurite-promoting activities, as did reduction and alkylation of disulfide bonds in the globular domain. Although cell adhesion and spreading were blocked by antibodies to an alpha 6 integrin subunit, neurite outgrowth was unaffected, indicating two distinct receptors for these two activities. Furthermore, a synthetic peptide, the sequence of which is found in the vicinity of adhesion and neurite-promoting sites and previously implicated in neurite growth and cell attachment activities, was found to be inactive. These results indicate that the major cell attachment and neurite-promoting sites of laminin are distinct although both require the native conformation of parts of the rod and the terminal globular domain of the long arm of laminin.     

Multiple cell surface receptors for the short arms of laminin: alpha 1 beta 1 integrin and RGD-dependent proteins mediate cell attachment only to domains III in murine tumor laminin

Cell surface molecules that interact with the cross formed by the three short arms of murine tumor laminin were studied using thermal perturbation, antibody and peptide blocking, and affinity chromatography. Several potential receptors for the laminin short arms were revealed that differed from those mediating cell attachment to the E8 (long arm) fragment. Two cell lines, Rugli and L8 attached well to E1-X (short arm) fragments of laminin. This attachment was blocked by antibodies against alpha 1 integrin chains. Other cells were unable to attach strongly to E1-X, but attached to P1. This attachment was unaffected by anti-beta 1 integrin antibodies, but specifically blocked by the peptide GRGDS. By contrast, binding of Rugli cells was RGD independent and blocked by anti-beta 1 integrin antibodies. G7 and C2C12 myoblasts were very sensitive to GRGDS (ID50 approximately 2 micrograms.ml-1) for attachment to P1 which implied that a non-beta 1 series integrin, possibly alpha v beta 3, was involved. On heat denaturation of P1(3) attachment remained sensitive to RGDS and ID50 was unchanged. On heat denaturation of E1-X, attachment remained sensitive to RGDS but the ID50 increased to approximately 200 micrograms.ml-1. Cellular beta 1 integrins were retained on laminin affinity columns. A beta 1 integrin with an approximately 190 kD alpha-chain could be isolated from Rugli cells whose attachment could be blocked by anti-alpha 1 antibodies and not from cells blocked by RGDS peptides. Anti-alpha 1 antibodies blocked Rugli attachment to native laminin, but only when the E8 cell binding sites on laminin were also blocked. Thus, a receptor related to alpha 1 beta 1 integrin can function simultaneously with a receptor for E8. Anti-alpha 1 also blocked attachment to heated laminin, suggesting that the heat-stable attachment activity in laminin involved the E1-X binding site. Thus, at least two putative receptors mediate attachment to the short arms of laminin. One, related to alpha 1 beta 1 integrin, recognizes RGDS-independent sites in E1-X defined by P1 (within domains III, IIIa, IIIb), and one is an RGD-dependent molecule recognizing sites in P1, and is not a beta 1 integrin.     

Identification of homologous biologically active sites on the N-terminal domain of laminin alpha chains.

Laminin, a multifunctional glycoprotein of the basement membrane, consists of three different subunits, alpha, beta, and gamma chains. To date, five different alpha chains have been identified. N-terminal domain VI in the alpha1 chain has various biological activities. Here we screened biologically active sequences on domain VI of the laminin alpha2, alpha3, and alpha5 chains using a large number of overlapping peptides. HT-1080 human fibrosarcoma cell attachment to the peptides was evaluated using peptide-coated plastic plates and peptide-conjugated Sepharose beads. We identified four cell adhesive sequences from laminin alpha2 chain domain VI, two sequences from the alpha3 chain, and two sequences from the laminin alpha5 chain. Sequences homologous to A13 (RQVFQVAYIIIKA, alpha1 chain 121-133) on all the alpha chains (FQIAYVIVKA, alpha2 chain 130-139; GQLFHVAYILIKF, alpha3 chain 96-108; FHVAYVLIKA, alpha5 chain 74-83) showed strong cell attachment activity. A5-16 (LENGEIVVSLVNGR, alpha5 chain 147-160) showed the strongest cell attachment activity in the plate assay, and the homologous peptide in the alpha3 chain promoted similar strong cell attachment activity. A5-16 and its homologous peptide in the alpha2 chain promoted moderate cell attachment, while the homologous peptide to A5-16 in the alpha1 chain did not show activity. A2-7 (SPSIKNGVEYHYV, alpha2 chain 108-120) showed cell attachment activity only in the plate assay, but homologous sequences in the alpha1, alpha3, and alpha5 chains did not promote activity. A2-7 promoted endothelial cell sprouting from aortic rings in vitro and melanoma colonization to murine lungs in vivo. However, none of the homologous peptides of A2-7 promoted experimental pulmonary metastasis by B16-BL6 melanoma cells. These results indicate that there are chain-specific active sites in domain VI of the laminin alpha chains, some of which contain conserved activities.     

Active peptides from the carboxyl-terminal globular domain of laminin α2 and Drosophila α chains

The laminin α1 chain carboxyl-terminal globular domain (G domain) contains multiple biological activities. Recently, we identified five cell binding sequences from the G domain by screening with overlapping 12-mer peptides encompassing the entire domain. The structures of these five sequences in the α1 chain are conserved in the corresponding regions of the different laminin α chains. Here we characterize the adhesion activities of the corresponding peptide segments from both the mouse laminin α2 chain and Drosophila laminin α chain using peptide-coated plastic plates and peptide-conjugated Sepharose beads. Using several cell lines, the laminin α2 chain peptides showed cell attachment and/or spreading activities with cell type specificities. Cell spreading on MG-10 was inhibited by integrin antibodies. Four of the Drosophila laminin peptides showed cell attachment activities. These results suggest that biologically active regions in the G domain are conserved in the laminin α1 and α2 chains, and that these regions in laminin play an important role in cell surface receptor interactions.     

Identification of active sequences in human laminin α5 G domain

Human laminin‐511 (α5β1γ1) and its truncated protein, laminin‐511 E8 fragment, bind to integrin α6β1 and have been widely used for embryonic stem cell and induced pluripotent stem cell culture under feeder‐free conditions. In this study, we focused on human laminin α5 chain G domain, which is thought to be critical for the biological functions of laminin‐511, and screened its biologically active sequences using a synthetic peptide library. We synthesized 115 peptides (hA5G1‐hA5G115) covering the entire laminin α5 chain G domain and evaluated cell attachment activity using both the peptide‐coated plate and peptide‐chitosan matrix (peptide‐ChtM) assays. Seventeen peptides demonstrated cell attachment activity in the assays. Both hA5G18 and hA5G26‐coated plates and hA5G74‐ChtMs promoted integrin β1‐mediated cell attachment. These findings are useful for the study of molecular mechanisms of laminin‐511, and the active peptides have a potential for use as a molecular probe for cell adhesion receptors.     

Identification of biologically active sequences in the laminin alpha 4 chain G domain

Laminins are a family of trimeric extracellular matrix proteins consisting of alpha, beta, and gamma chains. So far five different laminin alpha chains have been identified. The laminin alpha 4 chain, which is present in laminin-8/9, is expressed in cells of mesenchymal origin, such as endothelial cells and adipocytes. Previously, we identified heparin-binding sites in the C-terminal globular domain (G domain) of the laminin alpha 4 chain. Here we have focused on the biological functions of the laminin alpha 4 chain G domain and screened active sites using a recombinant protein and synthetic peptides. The rec-alpha 4G protein, comprising the entire G domain, promoted cell attachment activity. The cell attachment activity of rec-alpha 4G was completely blocked by heparin and partially inhibited by EDTA. We synthesized 116 overlapping peptides covering the entire G domain and tested their cell attachment activity. Twenty peptides showed cell attachment activity, and 16 bound to heparin. We further tested the effect of the 20 active peptides in competition assays for cell attachment and heparin binding to rec-alpha 4G protein. A4G6 (LAIKNDNLVYVY), A4G20 (DVISLYNFKHIY), A4G82 (TLFLAHGRLVFM), and A4G83 (LVFMFNVGHKKL), which promoted cell attachment and heparin binding, significantly inhibited both cell attachment and heparin binding to rec-alpha 4G. These results suggest that the four active sites are involved in the biological functions of the laminin alpha 4 chain G domain. Furthermore, rec-alpha 4G, A4G6, and A4G20 were found to interact with syndecan-4. These active peptides may be useful for defining of the molecular mechanism laminin-receptor interactions and laminin-mediated cellular signaling pathways.     

Syndecan binding sites in the laminin alpha1 chain G domain

The laminin alpha1 chain G domain has multiple biological activities. Previously, we identified cell binding sequences in the laminin alpha1 chain G domain by screening 113 synthetic peptide-polystyrene beads for cell attachment activity. Here, we have used a recombinant protein of the laminin alpha1 G domain (rec-alpha1G) and a large set of synthetic peptides to further identify and characterize heparin, cell, and syndecan-4 binding sites in the laminin alpha1 chain G domain. The rec-alpha1G protein promoted both cell attachment and heparin binding (K(D) = 19 nM). Cell attachment to the rec-alpha1G protein was inhibited 60% by heparin and 30% by EDTA. The heparin binding sites were identified by competing heparin binding to the rec-alpha1G protein with 110 synthetic peptides in solution. Only two peptides, AG73 (IC(50) = 147 microM) and AG75 (IC(50) = 206 microM), inhibited heparin binding to rec-alpha1G. When the peptides were compared in a solid-phase heparin binding assay, AG73 showed more heparin binding than AG75. AG73 also inhibited fibroblast attachment to the rec-alpha1G protein, but AG75 did not. Cell attachment to the peptides was studied using peptide-coated plates and peptide-conjugated sepharose beads. AG73 promoted cell attachment in both assays, but AG75 only showed cell attachment activity in the bead assay. Additionally, AG73, but not AG75, inhibited branching morphogenesis of mouse submandibular glands in organ culture. Furthermore, the rec-alpha1G protein bound syndecan-4, and both AG73 and AG75 inhibited this binding. These results suggest that the AG73 and AG75 sites are important for heparin and syndecan-4 binding in the laminin alpha1 chain G domain. These sites may play a critical role in the diverse biological activities involving heparin and syndecan-4 binding.     

Human basement membrane heparan sulfate proteoglycan core protein: a 467-kD protein containing multiple domains resembling elements of the low density lipoprotein receptor, laminin, neural cell adhesion molecules, and epidermal growth factor

The primary structure of the large human basement membrane heparan sulfate proteoglycan (HSPG) core protein was determined from cDNA clones. The cDNA sequence codes for a 467-kD protein with a 21-residue signal peptide. Analysis of the amino acid sequence showed that the protein consists of five domains. The amino-terminal domain I contains three putative heparan sulfate attachment sites; domain II has four LDL receptor-like repeats; domain III contains repeats similar to those in the short arms of laminin; domain IV has lg-like repeats resembling those in neural cell adhesion molecules; and domain V contains sequences resembling repeats in the G domain of the laminin A chain and repeats in the EGF. The domain structure of the human basement membrane HSPG core protein suggests that this mosaic protein has evolved through shuffling of at least four different functional elements previously identified in other proteins and through duplication of these elements to form the functional domains. Comparison of the human amino acid sequence with a partial amino acid sequence from the corresponding mouse protein (Noonan, D. M., E. A. Horigan, S. R. Ledbetter, G. Vogeli, M. Sasaki, Y. Yamada, and J. R. Hassell. 1988. J. Biol. Chem. 263:16379-16387) shows a major difference between the species in domain IV, which contains the Ig repeats: seven additional repeats are found in the human protein inserted in the middle of the second repeat in the mouse sequence. This suggests either alternative splicing or a very recent duplication event in evolution. The multidomain structure of the basement membrane HSPG implies a versatile role for this protein. The heparan sulfate chains presumably participate in the selective permeability of basement membranes and, additionally, the core protein may be involved in a number of biological functions such as cell binding, LDL-metabolism, basement membrane assembly, calcium binding, and growth- and neurite-promoting activities.     

Amino acid sequence of mouse nidogen, a multidomain basement membrane protein with binding activity for laminin, collagen IV and cells.

The whole amino acid sequence of nidogen was deduced from cDNA clones isolated from expression libraries and confirmed to approximately 50% by Edman degradation of peptides. The protein consists of some 1217 amino acid residues and a 28-residue signal peptide. The data support a previously proposed dumb-bell model of nidogen by demonstrating a large N-terminal globular domain (641 residues), five EGF-like repeats constituting the rod-like domain (248 residues) and a smaller C-terminal globule (328 residues). Two more EGF-like repeats interrupt the N-terminal and terminate the C-terminal sequences. Weak sequence homologies (25%) were detected between some regions of nidogen, the LDL receptor, thyroglobulin and the EGF precursor. Nidogen contains two consensus sequences for tyrosine sulfation and for asparagine beta-hydroxylation, two N-linked carbohydrate acceptor sites and, within one of the EGF-like repeats an Arg-Gly-Asp sequence. The latter was shown to be functional in cell attachment to nidogen. Binding sites for laminin and collagen IV are present on the C-terminal globule but not yet precisely localized.     

Attachment of cells to basement membrane collagen type IV

Of ten different cell lines examined, three showed distinct attachment and spreading on collagen IV substrates, and neither attachment nor spreading was enhanced by adding soluble laminin or fibronectin. This reaction was not inhibited by cycloheximide or antibodies to laminin, indicating a direct attachment to collagen IV without the need of mediator proteins. Cell-binding sites were localized to the major triple-helical domain of collagen IV and required an intact triple helical conformation for activity. Fibronectin showed preferential binding to denatured collagen IV necessary to mediate cell binding to the substrate. Fibronectin binding sites of collagen IV were mapped to unfolded structures of the major triple-helical domain and show a similar specificity to fibronectin-binding sites of collagen I. The data extend previous observations on biologically potential binding sites located in the triple helix of basement membrane collagen IV.     

Hepatocyte attachment to laminin is mediated through multiple receptors

The interaction of hepatocytes with the basement membrane glycoprotein laminin was studied using synthetic peptides derived from laminin sequences. Rat hepatocytes bind to laminin and three different sites within the A and B1 chains of laminin were identified. Active laminin peptides include the PA22-2 peptide (close to the carboxyl end of the long arm in the A chain), the RGD-containing peptide, PA21 (in the short arm of the A chain) and the pentapeptide YIGSR (in the short arm of the B1 chain). PA22-2 was the most potent peptide, whereas the other two peptides had somewhat lower activity. Furthermore, hepatocyte attachment to laminin was inhibited by the three peptides, with PA22-2 being the most active. Various proteins from isolated membranes of cell-surface iodinated hepatocytes bound to a laminin affinity column including three immunologically related binding proteins : Mr = 67,000, 45,000, and 32,000. Several proteins--Mr = 80,000, 55,000, and 38,000-36,000--with a lower affinity for laminin were also identified. Affinity chromatography on peptide columns revealed that the PA22-2 peptide specifically bound the Mr = 80,000, 67,000, 45,000, and 32,000 proteins, the PA21 peptide bound the Mr = 45,000 and 38,000-36,000 proteins and the YIGSR peptide column bound the 38,000-36,000 protein. Antisera to a bacterial fusion protein of the 32-kD laminin-binding protein (LBP-32) reacted strongly with the three laminin-binding proteins, Mr = 67,000, 45,000, and 32,000, showing that they are immunologically related. Immunoperoxidase microscopy studies confirmed that these proteins are present within the plasma membrane of the hepatocyte. The antisera inhibited the adhesion of hepatocytes to hepatocytes to laminin by 30%, supporting the finding that these receptors and others mediate the attachment of hepatocytes to several regions of laminin.     

Identification of a neurite outgrowth-promoting domain of laminin using synthetic peptides

We have identified a synthetic peptide derived from the B2-chain of mouse laminin, Arg-Asn-Ile-Ala-Glu-Ile-Ile-Lys-Asp-Ile (p20), which stimulates the neurite outgrowth-promoting activity of the native molecule. In organotypic cultures, neurons from newborn mouse brain or embryonic peripheral nervous system responded by extensive neurite outgrowth for native laminin or the peptide p20 in the culture medium. If rat cerebellar neurons were grown on laminin, 1-5 microM (1-5 micrograms/ml) of peptide p20 in the culture medium competed with laminin and inhibited neuronal attachment and neurite outgrowth, whereas higher concentrations (greater than 50 microM; greater than 50 micrograms/ml) had a specific neurotoxic effect. When peptide p20 was used as the culture substratum, neurite outgrowth in cerebellar cultures was up to 60% of that seen on native laminin. Our results indicate that a neurite outgrowth-promoting domain of laminin is located in the alpha-helical region of the B2-chain, and is active for both central and peripheral neurons.     

The complete sequence of perlecan, a basement membrane heparan sulfate proteoglycan, reveals extensive similarity with laminin A chain, low density lipoprotein-receptor, and the neural cell adhesion molecule.

A heparan sulfate proteoglycan is a component of all basement membranes. This molecule consists of three heparan sulfate side chains linked to a large core protein of approximately 400 kDa. We have isolated seven overlapping murine cDNA clones that encode the entire mRNA sequence of 12.685 kilobases of this molecule. This sequence has a single open reading frame of 3,707 amino acids that encodes for a protein of 396 kDa. Identical or near identical matchups with nine peptide sequences derived from the core protein of the molecule isolated from the Engelbreth-Holm-Swarm tumor were found with the deduced sequence. Sequence analysis and data base comparison of the deduced sequence show the protein to consist of five different domains, most of which contain internal repeats. Domain I contains a start methionine followed by a typical signal transfer sequence and a unique segment of 172 amino acids that contains the three probable sites of heparan sulfate attachment, SGD. Domain II contains four cysteine- and acidic amino acid-rich repeats that are very similar to those found in the LDL receptor and proteins such as GP330. Domain III consists of cysteine-rich and globular regions, both of which show similarity to those in the short arm of the laminin A chain. Domain IV contains 14 repeats of the immunoglobulin superfamily that are most highly similar to the immunoglobulin-like repeats in the neural cell adhesion molecule. Domain V contains three repeats with similarity to the laminin A chain G domain that are separated by epidermal growth factor-like regions not found in the laminin A chain. As the primary structural data agree with the appearance of the molecule in the electron microscope as a series of globules separated by rods, or "beads on a string," we have adopted the name perlecan for this molecule. The variety of domains in perlecan suggest multiple interactions with other molecules.     

Differential properties of attachment of human fibroblasts to various extracellular matrix proteins

Human diploid fibroblasts (TIG-3) were shown to attach and spread onto substrata coated with collagen, fibronectin, laminin and vitronectin. The cell attachment to these proteins required divalent cations. Mg2+ stimulated the cell attachment to all the proteins, while Ca2+ alone was not effective for the attachment to collagen and laminin. A mild trypsin treatment had prevented cells from attaching to the laminin, while it had no effect on the attachment to the other proteins. The fibronectin fragment, which retained cell binding activity, inhibited the cells from attaching and spreading onto fibronectin, but it did not cause any inhibition on the other proteins. The synthetic peptide GRGDSP inhibited the cells from attaching and spreading onto fibronectin and vitronectin, while it did not cause any inhibition on collagen and laminin. In attempts to isolate distinct receptors for these proteins, we were able to purify proteins very similar to the fibronectin and vitronectin receptors of human placenta. Based on the differential properties of the attachment of TIG-3 cells to these proteins and biochemical data, we indicate that human diploid fibroblasts have distinctive binding sites (receptors) for collagen, fibronectin, laminin and vitronectin.     

A cell attachment peptide from human C-reactive protein.

The serum acute phase reactant, C-reactive protein (CRP), is selectively deposited at sites of tissue damage and degraded by neutrophils into biologically active peptides. A synthetic peptide corresponding to residues 27-38 present in each of the five identical subunits of CRP mediated cell attachment activity in vitro. Although the CRP-derived peptide contains a Tuftsin (TKPR)-like sequence at its amino-terminus, the Tuftsin tetrapeptide itself, as well as several synthetic peptides of CRP, failed to inhibit the cell-attachment activity to the CRP-derived peptide. Peptides containing the sequences responsible for the cell attachment activity of the extracellular matrix proteins, fibronectin (Fn) and laminin, failed to inhibit the CRP-derived peptide cell attachment activity. However, the addition of the RGDS and RGDSPASSLP cell-binding peptides of Fn to cells enhanced attachment to the active peptide from CRP. In the converse experiment, the cell-binding peptide of CRP did not influence cell attachment to Fn or laminin. A peptide corresponding to the same stretch of amino acid residues within the homologous Pentraxin, serum amyloid P-component (SAP), displayed nearly identical cell-attachment activity. Several monoclonal antibodies (mAb) specific for the CRP-derived cell-binding peptide neutralized its cell-attachment activity. These mAbs reacted with intact CRP and neutralized the cell-binding activity of CRP itself. The findings suggest that a peptide with cell-binding activity could be generated from the breakdown of CRP and then contribute directly to cellular events leading to tissue repair.     

Cloning, expression, and spectroscopic characterization of Cucumis sativus stellacyanin in its nonglycosylated form.

The cDNA encoding the 182 amino acid long precursor stellacyanin from Cucumis sativus was isolated and characterized. The protein precursor consists of four sequence domains: I, a 23 amino acid hydrophobic N-terminal signal peptide with features characteristic of secretory proteins; II, a 109 amino acid copper-binding domain; III, a 26 amino acid hydroxyproline- and serine-rich peptide characteristic of motifs found in the extension family, extracellular structural glycoproteins found in plant cell walls; and IV, a 22 amino acid hydrophobic extension. Maturation of the protein involves posttranslational processing of domains I and IV. The copper-binding domain (domain II), which shares high sequence identity with other stellacyanins, has been expressed without its carbohydrate attachment sites, refolded from the Escherichia coli inclusion bodies, purified, and characterized by electronic absorption, EPR, ESEEM, and RR spectroscopy. Its spectroscopic properties are nearly identical to those of stellacyanin from the Japanese lacquer tree Rhus vernicifera, the most extensively studied and best characterized stellacyanin, indicating that this domain folds correctly, even in the absence of its carbohydrate moiety. The presence of a hydroxyproline- and serine-rich domain III suggests that stellacyanin may have a function other than that of a diffusible electron transfer protein, conceivably participating in redox reactions localized at the plant cell wall, which are known to occur in response to wounding or infection of the plant.     

A truncated laminin chain homologous to the B2 chain: structure, spatial expression, and chromosomal assignment

We describe the identification of a novel laminin chain. Overlapping clones were isolated from a human fibrosarcoma HT1080 cell cDNA library spanning a total of 5,200 bp. A second set of clones contained an alternative 3' end sequence giving a total of 4,316 bp. The longer sequence contained an open reading frame for a 1,193-residue-long polypeptide. The alternative sequence was shortened at the carboxyl-terminal end coding for a 1,111-residue-long polypeptide. The amino acid sequence contained 21 amino acids of a putative signal peptide and 1,172 residues or alternatively 1,090 residues of a sequence with five distinct domains homologous to domains I-V in laminin chains. Comparison of the amino acid sequences showed that the novel laminin chain is homologous to the laminin B2 chain. However, the structure of the novel laminin chain isolated here differs significantly from that of the B2 chain in that it has no domain VI and domains V, IV, and III are shorter, resulting in a truncated laminin chain. The alternative sequence had a shortened domain I/II. In accordance with the current nomenclature, the chain characterized here is termed B2t. Calculation of possible chain interactions of laminin chains with the B2t chain domain I/II indicated that the B2t chain can replace the B2 chain in some laminin molecules. The gene for the laminin B2t chain (LAMB2T) was localized to chromosome 1q25-q31 in close proximity to the laminin B2 chain gene. Northern analysis showed that the B2t chain is expressed in several human fetal tissues but differently from the laminin B1 and B2 chains. By in situ hybridization expression of the B2t chain was localized to specific epithelial cells in skin, lung, and kidney as opposed to a general epithelial and endothelial cell expression of the laminin B2 chain in the same tissues.     

Amino acid sequence and domain structure of entactin. Homology with epidermal growth factor precursor and low density lipoprotein receptor

Entactin (nidogen), a 150-kD sulfated glycoprotein, is a major component of basement membranes and forms a highly stable noncovalent complex with laminin. The complete amino acid sequence of mouse entactin has been derived from sequencing of cDNA clones. The 5.9-kb cDNA contains a 3,735-bp open reading frame followed by a 3'- untranslated region of 2.2 kb. The open reading frame encodes a 1,245- residue polypeptide with an unglycosylated Mr of 136,500, a 28-residue signal peptide, two Asn-linked glycosylation sites, and two potential Ca2+-binding sites. Analysis of the deduced amino acid sequence predicts that the molecule consists of two globular domains of 70 and 36 kD separated by a cysteine-rich domain of 28 kD. The COOH-terminal globular domain shows homology to the EGF precursor and the low density lipoprotein receptor. Entactin contains six EGF-type cysteine-rich repeat units and one copy of a cysteine-repeat motif found in thyroglobulin. The Arg-Gly-Asp cell recognition sequence is present in one of the EGF-type repeats, and a synthetic peptide from the putative cell-binding site of entactin was found to promote the attachment of mouse mammary tumor cells.     

Cell interactions with laminin and its proteolytic fragments during outgrowth of mouse primary trophoblast cells.

Mouse blastocysts in serum-free culture for 24-48 h become attachment-competent, adhere to fibronectin- or laminin-coated surfaces, and subsequently form trophoblast outgrowths. The blastocyst laminin receptor was characterized in outgrowth studies using modified laminin. Trophoblast cells interacted with the peptide portion of laminin, but not the oligosaccharide moiety since its adhesive activity was reduced by boiling or trypsin treatment, but not by treatments that removed or modified its carbohydrate. Laminin outgrowth-promoting activity was further localized within its structural domains by use of the well-characterized proteolytic fragments of laminin, E1-4, and E8, and a synthetic peptide, CDPGYIGSR. The E1-4 fragment of laminin did not promote embryo outgrowth. However, the E8 fragment, which contains a heparin-binding domain as well as sites recognized during cell adhesion and neurite outgrowth, vigorously promoted outgrowth in both the presence and absence of heparin, heparan sulfate, or heparinase. Consistent with these results, outgrowth on intact laminin was not inhibited by CDPGYIGSR, a sequence within the E1-4 fragment that is known to mediate the adhesion of some cell types. It is concluded from these results that early trophoblast cells adhere to peptide in the E8 domain of laminin using a mechanism that is independent of the one used for adhesion to fibronectin.