Structure of the human laminin B2 chain gene reveals extensive divergence from the laminin B1 chain gene

The exon-intron structure of the human laminin B2 chain gene was elucidated from genomic lambda phage clones spanning 2 kilobase pairs (kb) of the 5'-flanking region, 58 kb of the structural gene and 10 kb of the 3'-flanking region. The entire gene was shown to contain 28 exons. The promoter region has no TATA or CAAT boxes whereas it contains five GC boxes and three AP-2-like binding sites. Comparison with the promoter region of the mouse gene revealed six highly conserved sequences of 14 to 42 base pairs in length. Sequencing of the last exon of the gene showed that the 3'-untranslated region of the mRNA can be up to 2797 nucleotides with five AATAAA potential polyadenylation signals. The similarity of the human 3'-untranslated sequence with that of mouse was shown to be 68.8%. The exon-intron structure of the laminin B2 chain gene demonstrated extensive divergence from the human laminin B1 chain gene, which has 34 exons. Only three intron locations are conserved in these two genes. The overall exon profile of the laminin B2 chain gene correlates only marginally with the pattern of structural domains and internal cysteine-rich repeats in the laminin B2 polypeptide chain.     

Evidence for coiled-coil alpha-helical regions in the long arm of laminin.

Three new laminin fragments, E8, E9 and 25K with mol. wt. 50 000-280 000, were prepared from a limited elastase digest of laminin and from tissue extracts. They were similar with respect to their rod-like structure, a high alpha-helix content, the assembly from two chain segments and immunological cross-reactivity. Two of the fragments (E8 and E9) possess in addition globular domains which lack alpha-helices. Chemical, immunological and physical data together with sequence analysis strongly indicate that the alpha-helical segments are assembled in coiled-coil structures which are located in the rod of the long arm of laminin. These data give new insights into the overall structure of the protein.     

Monoclonal anti-mouse laminin antibodies: AL-1 reacts with laminin alpha1 chain, AL-2 with laminin beta1 chain, and AL-4 with the coiled-coil domain of laminin beta1 chain.

We analyzed the reactivity of three different commercially available rat monoclonal antibodies raised against mouse laminin-alpha1beta1gamma1 (laminin-111), AL-1, AL-2, and AL-4. Using ELISA assays, Western blot analysis and immunostainings we present refined epitope maps for these three laminin monoclonals. AL-1 reacted, as predicted with laminin alpha1 chain. AL-4 has also been marketed as an alpha1 chain specific probe, but we show here that AL-4 detects mouse laminin beta1 chain, in the distal part of the coiled-coil region. AL-2 was predicted to react with all three chains near the cross-region, but seems to primarily react with laminin beta1 chain.     

Interaction of agrin with laminin requires a coiled-coil conformation of the agrin-binding site within the laminin gamma1 chain

Coiled-coil domains are found in a wide variety of proteins, where they typically specify subunit oligomerization. Recently, we have demonstrated that agrin, a multidomain heparan sulfate proteoglycan with a crucial role in the development of the nerve-muscle synapse, binds to the three-stranded coiled-coil domain of laminin-1. The interaction with laminin mediates the integration of agrin into basement membranes. Here we characterize the binding site within the laminin-1 coiled coil in detail. Binding assays with individual laminin-1 full-length chains and fragments revealed that agrin specifically interacts with the gamma1 subunit of laminin-1, whereas no binding to alpha1 and beta1 chains was detected. By using recombinant gamma1 chain fragments, we mapped the binding site to a sequence of 20 residues. Furthermore, we demonstrate that a coiled-coil conformation of this binding site is required for its interaction with agrin. The finding that recombinant gamma1 fragments bound at least 10-fold less than native laminin-1 indicates that the structure of the three-stranded coiled-coil domain of laminin is required for high-affinity agrin binding. Interestingly, no binding to a chimeric gamma2 fragment was observed, indicating that the interaction of agrin with laminin is isoform specific.     

Human laminin B2 chain. Comparison of the complete amino acid sequence with the B1 chain reveals variability in sequence homology between different structural domains

The complete amino acid sequence of the human laminin B2 chain has been determined by sequencing of cDNA clones. The six overlapping clones studied cover approximately 7.5 kilobases of which 5312 nucleotides were sequenced from the 5' end. The open reading frame codes for a 33-residue signal peptide and a 1576-residue B2 chain proper, which is 189 residues less than in the highly homologous B1 chain (Pikkarainen, T., Eddy, R., Fukushima, Y., Byers, M., Shows, T., Pihlajaniemi, T., Saraste, M., and Tryggvason, K. (1987) J. Biol. Chem. 262, 10454-10462). Computer analysis revealed that the B2 chain consists of distinct domains that contain helical structures, cysteine-rich repeats, and globular regions, as does the B1 chain. However, domain alpha and domain beta of the B1 chain have no counterpart in B2, and the number of cysteine-rich repeats is 12, or 1 less than in the B1 chain. The degree of homology between the two chains is highest in the cysteine repeat-containing domains III and V where 40% of the residues match. However, results demonstrate that the B1 and B2 chains of laminin are highly homologous proteins that are probably the products of related genes.     

Primary structure of the mouse laminin B2 chain and comparison with laminin B1

One of the major components of basement membranes is the glycoprotein laminin, made up of three disulfide-bonded subunits, the A, B1, and B2 chains. We have isolated and sequenced overlapping mouse laminin B2 chain cDNA clones covering 7562 base pairs. The deduced amino acid sequence predicts that the mature B2 chain consists of 1572 residues, has an unglycosylated molecular weight of 173,541, and possesses 14 potential N-linked glycosylation sites. Analysis of the predicted secondary structure shows the presence of six domains, two rich in alpha-helical structure, two composed of homologous cysteine-rich repeat units, and two globular regions. The organization of the molecule is very similar to that of the mouse laminin B1 chain, and significant sequence homology between the B1 and B2 chains was found in their two cysteine-rich domains and in their amino-terminal globular domains.     

Drosophila substrate adhesion molecule: sequence of laminin B1 chain reveals domains of homology with mouse

Laminin, a substrate adhesion molecule in vertebrates, is a large glycoprotein complex in basement membranes that promotes cell adhesion, cell migration, and neurite outgrowth. Here we report on the cloning of the genes encoding the three subunits of Drosophila laminin. Sequence analysis of cDNA clones encoding the Drosophila B1 chain reveals a multidomain structure similar to that of its mouse homolog. The Drosophila sequence has only 25% amino acid identity with the mouse sequence in domains I, II, and IV. However, in one of the putative collagen-binding regions (domain VI) and the two cysteine-rich domains of EGF-like repeats (domains III and V), the amino acid identity between these two evolutionarily distant species jumps to 55%. Moreover, the number, length, and unique amino acid sequences of each of the 13 EGF-like repeats are highly conserved between Drosophila and mouse, suggesting that each may serve a unique function in protein-protein interactions.     

A novel laminin B1 chain variant in avian eye.

Two distinct recombinant cDNA clones having homology to mouse laminin B1 have been isolated from an adult chicken eye library by cross-species nucleic acid hybridization. DNA sequence analysis identified one cDNA as the chicken homologue of the prototypic EHS laminin B1 chain. The second recombinant cDNA encodes a portion of a laminin B1-like protein, which is neither the chicken homologue of laminin B1 nor s-laminin, and thus represents a new laminin B1 variant.     

Synthesis and conformation of the trimeric coiled-coil segment of laminin.

A disulfide linked 95-mer parallel hetero-trimeric active site segment of laminin was designed and synthesised. The three subunits, A (32-mer), B1 (30-mer) and B2 (33-mer), were prepared by Boc-based solid-phase peptide synthesis involving a two-step trimethylsilyl bromide-thioanisole and HF deprotection procedure. The interlinking of the three subunits was accomplished by the stepwise selective formation of two disulfide bridges using air-oxidation and thallium (III) trifluoroacetate oxidation. The conformations of the synthetic peptides were studied by circular dichroism (CD) spectroscopy, showing that the hetero-dimer, B1-B2, one of the homo-dimers, B1-B1, and the trimer are 30 to 40% in the alpha-helical conformation in aqueous buffer. Variable temperature CD studies demonstrated that the trimer is considerably more stable (melting temperature (Tm) = 61 degrees) than the hetero-dimer, B1-B2 (Tm = 36 degrees).     

The laminin B2 chain has a multidomain structure homologous to the B1 chain

Laminin (Mr = 850,000) is a large basement membrane-specific glycoprotein composed of three chains: A, B1, and B2. Previously, we have reported the primary structure of the B1 chain of mouse laminin deduced from sequencing cDNA clones (Sasaki M., Kato, S., Kohno, K., Martin, G. R., and Yamada, Y. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 935-939). Here we report the isolation of overlapping cDNA clones spanning 7642 bases which encode the entire B2 chain. The nucleotide sequence of the clones contains an open reading frame of 4821 bases coding for a protein of 1607 amino acids including 33 amino acids of a presumptive signal peptide. The mRNA for the B2 chain contains 2.5 kilobases of 3'-untranslated region. The deduced amino acid sequence indicates that the B2 chain consists of six distinct domains, including two domains with alpha-helical, coiled-coil structures, two domains with cysteine-rich homologous repeats, and two globular domains. These structural features of the B2 chain are similar to those of the B1 chain. In addition, the amino acid sequences of the B2 and B1 chains demonstrate considerable homology, suggesting that the genes for these two chains arose from a common ancestor.     

The intrinsic factor-vitamin B12 receptor, cubilin, is assembled into trimers via a coiled-coil alpha-helix

A large protein was purified from bovine kidney, using selective extraction with EDTA to solubilize proteins anchored by divalent cation-dependent interactions. An antiserum raised against the purified protein labeled the apical cell surface of the epithelial cells in proximal tubules and the luminal surface of small intestine. Ten peptide sequences, derived from the protein, all matched the recently published sequences for rat (Moestrup, S. K., Kozyraki, R., Kristiansen, M., Kaysen, J. H., Holm Rasmussen, H., Brault, D., Pontillon, F., Goda, F. O., Christensen, E. I., Hammond, T. G., and Verroust, P. J. (1998) J. Biol. Chem. 273, 5235-5242) and human cubilin, a receptor for intrinsic factor-vitamin B12 complexes, identifying the protein as bovine cubilin. In electron microscopy, a three-armed structure was seen, indicating an oligomerization of three identical subunits. This model was supported by the Mr values of about 1,500,000 for the intact protein and 440,000 for its subunits obtained by analytical ultracentrifugation. In a search for a potential assembly domain, we identified a region of heptad repeats in the N-terminal part of the cubilin sequence. Computer-assisted analysis supported the presence of a coiled-coil alpha-helix between amino acids 103 and 132 of the human cubilin sequence and predicted the formation of a triple coiled-coil. We therefore conclude that cubilin forms a noncovalent trimer of identical subunits connected by an N-terminal coiled-coil alpha-helix.     

Structure of the C-terminal laminin G-like domain pair of the laminin alpha2 chain harbouring binding sites for alpha-dystroglycan and heparin

The laminins are large heterotrimeric glycoproteins with fundamental roles in basement membrane architecture and function. The C-terminus of the laminin alpha chain contains a tandem of five laminin G-like (LG) domains. We report the 2.0 A crystal structure of the laminin alpha2 LG4-LG5 domain pair, which harbours binding sites for heparin and the cell surface receptor alpha-dystroglycan, and is 41% identical to the laminin alpha1 E3 fragment. LG4 and LG5 are arranged in a V-shaped fashion related by a 110 degrees rotation about an axis passing near the domain termini. An extended N-terminal segment is disulfide bonded to LG5 and stabilizes the domain pair. Two calcium ions, one each in LG4 and LG5, are located 65 A apart at the tips of the domains opposite the polypeptide termini. An extensive basic surface region between the calcium sites is proposed to bind alpha-dystroglycan and heparin. The LG4-LG5 structure was used to construct a model of the laminin LG1-LG5 tandem and interpret missense mutations underlying protein S deficiency.     

Biological activities of homologous loop regions in the laminin alpha chain G domains

Laminin alpha chains (alpha1-alpha5 chains) have diverse chain-specific biological functions. The LG4 modules of laminin alpha chains consist of a 14-stranded beta-sheet (A-N) sandwich structure. Several biologically active sequences have been identified in the connecting loop regions. Here, we evaluated the biological activities of the loop regions of the E and F strands in the LG4 modules using five homologous peptides from each of the mouse alpha chains (EF-1: DYATLQLQEGRLHFMFDLG, alpha1 chain 2747-2765; EF-2: DFGTVQLRNGFPFFSYDLG, alpha2 chain 2808-2826; EF-3: RDSFVALYLSEGHVIFALG, alpha3 chain 2266-2284; EF-4: DFMTLFLAHGRLVFMFNVG, alpha4 chain 1511-1529; EF-5: SPSLVLFLNHGHFVAQTEGP, alpha5 chain 3304-3323). These homologous peptides showed chain-specific cell attachment and neurite outgrowth activities. Well organized actin stress fibers and focal contacts with vinculin accumulation were observed in fibroblasts attached on EF-1, whereas fibroblasts on EF-2 and EF-4 showed filopodia with ruffling. Fibroblast attachment to EF-2 and EF-4 was mediated by syndecan-2. In contrast, EF-1 promoted alpha2beta1 integrin-mediated fibroblast attachment and inhibited fibroblast attachment to a recombinant laminin alpha1 chain LG4-5. The receptors for EF-3 and EF-5 are unknown. Further, when the active core sequence of EF-1 was cyclized, utilizing two additional cysteine residues at both the N and C termini through a disulfide bridge, the cyclic peptide significantly enhanced integrin-mediated cell attachment. These results indicate that integrin-mediated cell attachment to the EF-1 sequence is conformation-dependent and that the loop structure is important for the activity. The homologous peptides, which promote either integrin- or syndecan-mediated cell attachment, may be useful for understanding the cell type- and chain-specific biological activities of the laminins.     

Nature of amino acid side chain and alpha-helix stability.

In order to investigate the ability of neutral amino acids to support the α-helix conformation, the coil–helix transition of poly(L -lysine) and of lysine copolymers with these amino acids was studied in water/methanol using circular dichroism. The transtions were recorded at constant pH adding buffer to the methanol/water mixtures. With poly(L -lysine), experiments were performed at several constant pH's; the transition midpoint on the water (methanol) concentration scale was found to depend strongly upon pH; the helix stability region is shifted towards higher water concentrations, when the pH is increased. Copolymers of lysine and several neutral amino acids revealed the same effect in that increasing amounts of, for example, norleucine also shifted the transition midpoint to higher water concentrations. A series of copolymers containing L -lysine as the host and different hydrophobic amino acids were synthesized and the helix–coil transition in water/methanol was observed at constant pH. Different copolymers of equal composition showed significant differences with respect to the nature of the amino acid incorporated into polylysine. From these studies an α-helix-philic scale (in decreasing order): Leu, Nle, Ile, Ala, Phe, Val, Gly is deduced and discussed; the results obtained were compared with those of different procedures.     

Importance of the solvent-exposed residues of the insulin B chain alpha-helix for receptor binding

Conjointly, the solvent-exposed residues of the central alpha-helix of the B chain form a well-defined ridge, which is flanked and partly overlapped by the two described insulin receptor binding surfaces on either side of the insulin molecule. To evaluate the importance of this interface in insulin receptor binding, we developed a new powerful method that allows us to introduce all the naturally occurring amino acids into a given position and subsequently determine the receptor binding affinities of the resulting insulin analogues. The total amino acid scanning mutagenesis was performed at positions B9, B10, B12, B13, B16, and B17, and the vast majority of the insulin analogue precursors were expressed and secreted in amounts close to that of the wild-type (human insulin) precursor. The analogue binding data revealed that positions B12 and B16 were the two positions most affected by the amino acid substitutions. Interestingly, the receptor binding affinities of the B13 analogues were also markedly affected by the amino acid substitutions, suggesting that GluB13 indeed is a part of insulin's binding surface. The B10 library screen generated analogues covering a wide range of (20-340%) of relative binding affinities, and the results indicated that a structural stabilization of the central alpha-helix and thereby a more rigid presentation of the binding epitope at the insulin receptor is important for receptor recognition. In conclusion, systematic amino acid scanning mutagenesis allowed us to confirm the importance of the B chain alpha-helix as a central recognition element serving as a linker of a continual binding surface.     

A pentapeptide from the laminin B1 chain mediates cell adhesion and binds the 67,000 laminin receptor

Laminin promotes epithelial cell adhesion in part through a site of nine amino acids CDPGYIGSR on the B1 chain. Using smaller synthetic peptides from this sequence as well as various peptides with amino acid substitutions, we find that the minimum sequence necessary for efficient cell adhesion as well as receptor binding is YIGSR. The deletion of tyrosine or the substitution of arginine in the peptides resulted in a significant loss of activity. The presence of an amide group on the terminal arginine of either peptide increases activity significantly. YIGSR is active in promoting the adhesion of a variety of epithelial cells; however, it is inactive with chondrocytes, fibroblasts, and osteoblasts.