Formation of the 67-kDa laminin receptor by acylation of the precursor

Even though the involvement of the 67-kDa laminin receptor (67LR) in tumor invasiveness has been clearly demonstrated, its molecular structure remains an open problem, since only a full-length gene encoding a 37-kDa precursor protein (37LRP) has been isolated so far. A pool of recently obtained monoclonal antibodies directed against the recombinant 37LRP molecule was used to investigate the processing that leads to the formation of the 67-kDa molecule. In soluble extracts of A431 human carcinoma cells, these reagents recognize the precursor molecule as well as the mature 67LR and a 120-kDa molecule. The recovery of these proteins was found to be strikingly dependent upon the cell solubilization conditions: the 67LR is soluble in NP-40-lysis buffer whereas the 37LRP is NP-40-insoluble. Inhibition of 67LR formation by cerulenin indicates that acylation is involved in the processing of the receptor. It is likely a palmitoylation process, as indicated by sensitivity of NP-40-soluble extracts to hydroxylamine treatment. Immunoblotting assays performed with a polyclonal serum directed against galectin3 showed that both the 67- and the 120-kDa proteins carry galectin3 epitopes whereas the 37LRP does not. These data suggest that the 67LR is a heterodimer stabilized by strong intramolecular hydrophobic interactions, carried by fatty acids bound to the 37LRP and to a galectin3 cross-reacting molecule. J. Cell. Biochem. 69:244–251, 1998. © 1998 Wiley-Liss, Inc.     

Cloning and characterization of full-length coding sequence (CDS) of the ovine 37/67-kDa laminin receptor (<Emphasis Type="Italic">RPSA</Emphasis>)

The 37-kDa Laminin Receptor Precursor (LRP)/67-kDa Laminin Receptor (LR), also known as ribosomal protein SA (RPSA), had been identified as a putative cell surface receptor for prions. Herein, we isolated the full-length coding sequence (CDS) of the ovine 37/67-kDa LRP/LR gene and submitted it to the GenBank under accession number EF649775. The open reading frame (ORF) of the 37/67-kDa LRP/LR CDS is 885 bp in length, containing six exons encoding a protein of 295 amino acids. The nucleotide sequence presented here is well coincided with the whole ovine genome of the 37/67-kDa LRP/LR previously published. Moreover, we identified four novel single nucleotide polymorphism sites (SNPs) at position 324 in exon 4, positions at 809, 875, and 881 in exon 7, respectively. Further, based on the deduced amino acid sequence alignment of the 37/67-kDa LRP/LR from human, cattle, mice, pig, chicken, and sheep, we also identified three polymorphic amino acid sites (PAAs) at residues 241, 272, and a novel site at residue 270 in the putative indirect prion protein (PrP) interaction region (180–285) on 37/67-kDa LRP/LR. Prediction of protein secondary structure further indicated that PAAs at residues 241, 270 and 272 may cause protein conformation changes as predicted, which may affect on the binding with prion protein. In addition, multiple-tissues RT-PCR results revealed that 37/67-kDa LRP/LR mRNA is expressed in all the 11 selected ovine tissues.     

Polymorphism of Fecundity Genes (FecB,FecX, and FecG) in the Indian Bonpala Sheep

The 37-kDa laminin receptor precursor/67-kDa laminin receptor (LRP/LR, also known as ribosomal protein SA, RPSA) has been reported to be involved in cancer development and prion internalization. Previous studies have shown that the LRP/LR is expressed in a wide variety of tissues. In particular, expression of LRP/LR mRNA may be closely related to the degree of PrP^Sc propagation. This study presents a detailed investigation of the LRP/LR mRNA expression levels in eleven normal ovine tissues. Using real-time quantitative PCR, the highest LRP/LR expression was found in neocortex (p < 0.05). Slightly lower levels were found in the heart and obex. Intermediate levels were seen in hippocampus, cerebellum, spleen, thalamus, mesenteric lymph node, and the lowest levels were present in liver, kidney, and lung. In general, the LRP/LR mRNA levels were much higher in neuronal tissues than in peripheral tissues. The observation that differences in LRP/LR mRNA expression levels are consistent with the corresponding variation in PrP^Sc accumulation suggests that the 37-kDa/67-kDa laminin receptor may be involved in the regulation of PrP^Sc propagation.     

siRNA-mediated silencing of the 37/67-kDa high affinity laminin receptor in Hep3B cells induces apoptosis

The laminin-binding protein, variously called the 37/67-kDa high affinity laminin receptor or p40, mediates the attachment of normal cells to the laminin network, and also has a role as a ribosomal protein. Over-expression of this protein has been strongly correlated with the metastatic phenotype. However, few studies have investigated the cellular consequence of the ablation of this gene's expression. To address this issue, the expression of the 37/67-kDa high affinity laminin receptor was knocked out with several siRNA constructs via RNA interference in transformed liver (Hep3B) cells. In each case where the message was specifically ablated, apoptosis was induced, as determined by annexin V/propidium iodide staining, and by double staining with annexin V and an antibody directed against the 37/67-kDa high affinity laminin receptor. These results suggest that this protein plays a critical role in maintaining cell viability.     

Cell Localization and Redistribution of the 67 kD Laminin Receptor and α6βl Integrin Subunits in Response to Laminin Stimulation: An Immunogold Electron Microscopy Study

The 67 kDa laminin receptor (67LR), one of several cell surface laminin-binding proteins, is involved in the interactions between cancer cells and laminin during tumor invasion and metastasis. A 37 kDa polypeptide (37LRP), previously identified as the 67LR precursor, is abundantly present in the cytoplasm and has been implicated in polysome formation. To better understand the cellular localization of the 67LR and its precursor, transmission electron microscopic studies of human melanoma A2058 cells were carried out using immunogold labeling and a variety of antibodies: (a) affinity purified antibodies directed against 37LRP cDNA-derived synthetic peptides; (b) anti-67LR monoclonal antibodies raised against intact human small cell lung carcinoma cells; and (c) monoclonal antibodies against the subunits of the integrin laminin receptor, α6β1. Double-labeling immunocyto-chemistry revealed that anti-67LR monoclonal antibodies as well as anti-37 LRP antibodies recognized antigens that were localized in the cytoplasm in electron dense structures. As expected, cell membrane labeling was also observed. Surprisingly, α6 and β1 integrin subunits were detected in the same cytoplasmic structures positive for the 67LR and the 37LRP. After addition of soluble laminin to A2058 cells in suspension, the number of labeled cytoplasmic structures increased especially in the vicinity of the plasma membrane, and were exported onto the cell surface. Neither fibronectin nor BSA induced such an effect. The data demonstrate that the 67 LR and the 37 LRP antibodies detect colocalized antigens that are in cytoplasmic structures with α6β1 integrin. These laminin binding protein rich structures could potentially form a supply of receptors that are exported to the surface upon exposure of the cells to laminin, with a consequent increase in the number of binding sites for the ligand. This system could define a mechanism through which cancer cells modulate their interaction with laminin.     

Characterization of the porcine multicopy ribosomal protein SA/37-kDa laminin receptor gene family

Prions represent a new class of infectious agents. The pathogenic prion protein (PrPSc) is known as the trigger of bovine transmissible spongiform encephalopathy (TSE). By contrast, an oral transmission of PrPSc and an ensuing infection seems to be blocked in non-ruminants such as pigs. Several investigations postulate that the ribosomal protein SA (RPSA) previously named 37-kDa laminin receptor precursor (LRP)/67-kDa laminin receptor (LR) is the candidate for binding and internalization of externally added cellular prion protein in the gut. We isolated a porcine ribosomal protein SA cDNA that consists of 1064 bp with an open reading frame of 885 bp encoding a 295 aa protein. The alignment of vertebrate ribosomal protein SA sequences displayed interspecies differences between cattle and pigs at positions 241 and 272 in the putative indirect PrP interaction site (aa 180-285) on RPSA. A PAC library screen revealed the existence of two processed ribosomal protein SA pseudogenes (RPSAP1 and RPSAP3) and of one non-processed pseudogene (RPSAP2). The pseudogenes have been assigned to SSC6 and SSC1 by hybrid panel analyses and FISH. Compared with the porcine cDNA 3, 7, and 13 insdels, 36, 25, and 57 single nucleotide exchanges and 6, 10, and 8 premature stop codons have been deciphered for RPSAP1, RPSAP2, and RPSAP3. In the 5', 3', and intron like regions, 2 (RPSAP1), 10 (RPSAP2), and 4 (RPSAP3) repeats have been detected. Basically, the repeats belong to one of the class/family LINE/L1, SINE/tRNA-Glu and DNA/MER1_type. We conclude that the pig genome contains multiple copies of the RPSA sequence probably as a consequence to maintain the multifunctionality of the mature protein.     

Biosynthesis of the 67 kDa high affinity laminin receptor.

High affinity interactions between cells and laminin are mediated, at least in part, by the 67 kDa laminin receptor (67 LR). A 37 kDa nascent polypeptide (37 LRP), predicted by a full length cDNA clone and obtained by in vitro translation of hybrid-selected laminin receptor mRNA, has been immunologically identified in cancer cell extracts as the putative precursor of the 67 LR. In this study, we used affinity purified antibodies developed against cDNA-deduced 37 LRP synthetic peptides in pulse chase experiments and demonstrated a precursor-product relationship between the 37 LRP and the 67 LR. Immunoblot, pulse chase and immunofluorescence experiments showed that transient transfection of the full length 37 LRP cDNA clone induced a dramatic increase in the synthesis of the 37 LRP but not of the mature 67 LR. We propose that the 67 LR results from the association of two gene products: the 37 LRP and a polypeptide yet to be identified.     

Knock-Down of the 37-kDa/67-kDa Laminin Receptor in Mouse Brain by Transgenic Expression of Specific Antisense LRP RNA

The 37-kDa/67-kDa laminin receptor (LRP/LR) plays a major role in the propagation of PrPSc, the abnormal form of the prion protein. In order to ablate the expression of LRP/LR in mouse brain we generated transgenic mice ectopically expressing antisense LRP RNA in the brain under control of the neuron-specific enolase (NSE) promoter. Hemizygous transgenic mice TgN(NSEasLRP)2 showed a significant reduction of LRP/LR protein levels in hippocampal and cerebellar brain regions. These mice might act as powerful tools to investigate the role of the laminin receptor in scrapie pathogenesis.     

The 37-kDa/67-kDa laminin receptor acts as the cell-surface receptor for the cellular prion protein.

Recently, we identified the 37-kDa laminin receptor precursor (LRP) as an interactor for the prion protein (PrP). Here, we show the presence of the 37-kDa LRP and its mature 67-kDa form termed high-affinity laminin receptor (LR) in plasma membrane fractions of N2a cells, whereas only the 37-kDa LRP was detected in baby hamster kidney (BHK) cells. PrP co-localizes with LRP/LR on the surface of N2a cells and Semliki Forest virus (SFV) RNA transfected BHK cells. Cell-binding assays reveal the LRP/LR-dependent binding of cellular PrP by neuronal and non-neuronal cells. Hyperexpression of LRP on the surface of BHK cells results in the binding of exogenous PrP. Cell binding is similar in PrP(+/+) and PrP(0/0) primary neurons, demonstrating that PrP does not act as a co-receptor of LRP/LR. LRP/LR-dependent internalization of PrP is blocked at 4 degrees C. Secretion of an LRP mutant lacking the transmembrane domain (aa 86-101) from BHK cells abolishes PrP binding and internalization. Our results show that LRP/LR acts as the receptor for cellular PrP on the surface of mammalian cells.     

Characterization of a putative clone for the 67-kilodalton elastin/laminin receptor suggests that it encodes a cytoplasmic protein rather than a cell surface receptor

The 67-kDa elastin binding protein shares many immunological and structural properties with the high-affinity 67-kDa tumor cell laminin receptor. Taking advantage of these similarities, we have screened a bovine cDNA library with a partial cDNA probe for the laminin receptor and have isolated and characterized a cDNA clone of 1038 bp that hybridizes to a single-size mRNA of 1.3 kb. The clone encodes a protein with a predicted molecular weight of 33K that lacks an N-terminal leader sequence, shows no posttranslational processing when translated in vitro in the presence of microsomes, and does not bind to elastin affinity columns. Although the bovine clone is nearly identical with clones encoding human and mouse proteins proported to be 67-kDa laminin receptor, physical and functional characteristics of the encoded protein suggest that it is a cytoplasmic protein that does not bind elastin. This finding calls into question the earlier conclusion that the clone encodes the 67-kDa receptor.     

37-kDa laminin receptor precursor mediates GnRH-II-induced MMP-2 expression and invasiveness in ovarian cancer cells

GnRH-II enhances ovarian cancer cell invasion in an autocrine manner. We have now found that GnRH-II increases 37-kDa laminin receptor precursor (LRP) production in GnRH receptor (GnRHR)-positive OVCAR-3 and CaOV-3 ovarian cancer cells, while small interfering RNA (siRNA)-mediated depletion of GnRH-II or GnRHR mRNA abrogates this. The invasiveness of ovarian cancer cells is also reduced >85% by siRNA-mediated knockdown of LRP levels and >50% by pretreatment of Matrigel with a synthetic peptide that blocks interactions between laminin and the 67-kDa nonintegrin laminin receptor which comprises two LRP subunits. Conversely, overexpressing LRP in CaOV-3 cells increases their invasiveness 5-fold, while overexpressing LRP with a nonfunctional laminin-binding site does not. Depletion of LRP by siRNA treatment reduces CaOV-3 cell attachment to laminin-coated plates by ～80% but only reduces their binding to Matrigel by ～20%. Thus, while LRP influences CaOV-3 cell adhesion to laminin, LRP must act in other ways to enhance invasion. Matrix metalloproteinases (MMPs) are key mediators of invasion, and LRP siRNA treatment of OVCAR-3 and CaOV-3 cells inhibits MMP-2 but not MMP-9 mRNA levels. Overexpressing LRP in these cells increases MMP-2 production specifically, while a laminin-binding deficient LRP does not. Importantly, LRP siRNA treatment abolishes GnRH-II-induced MMP-2 production, and invasion in OVCAR-3 and CaOV-3 cells, which was also seen after MMP-2 siRNA treatment. These results suggest that GnRH-II-induced LRP expression increases the amount of the 67-kDa nonintegrin laminin receptor, which appears to interact with laminin in the extracellular matrix to promote MMP-2 expression and enhance ovarian cancer cell invasion.     

The 67 kDa laminin receptor: structure, function and role in disease

The 67LR (67 kDa laminin receptor) is a cell-surface receptor with high affinity for its primary ligand. Its role as a laminin receptor makes it an important molecule both in cell adhesion to the basement membrane and in signalling transduction following this binding event. The protein also plays critical roles in the metastasis of tumour cells. Isolation of the protein from either normal or cancerous cells results in a product with an approx. molecular mass of 67 kDa. This protein is believed to be derived from a smaller precursor, the 37LRP (37 kDa laminin receptor precursor). However, the precise mechanism by which cytoplasmic 37LRP becomes cell-membrane-embedded 67LR is unclear. The process may involve post-translational fatty acylation of the protein combined with either homo- or hetero-dimerization, possibly with a galectin-3-epitope-containing partner. Furthermore, it has become clear that acting as a receptor for laminin is not the only function of this protein. 67LR also acts as a receptor for viruses, such as Sindbis virus and dengue virus, and is involved with internalization of the prion protein. Interestingly, unmodified 37LRP is a ribosomal component and homologues of this protein are found in all five kingdoms. In addition, it appears to be strongly associated with histones in the eukaryotic cell nucleus, although the precise role of these interactions is not clear. Here we review the current understanding of the structure and function of this molecule, as well as highlighting areas requiring further research.     

Seventeen copies of the human 37 kDa laminin receptor precursor/p40 ribosome-associated protein gene are processed pseudogenes arisen from retropositional events

A cDNA coding for a 37 kDa polypeptide has been identified in several species as both the potential precursor of the 67 kDa laminin receptor (37LRP) and a putative ribosome-associated protein (p40). Interestingly, increased expression of this polypeptide (37LRP/p40) is consistently observed in invasive and metastatic cancer cells and is associated with poor prognosis. Southern-blot analysis of human genomic DNA predicted multiple copies of the 37LRP/p40 gene. In this study, we report that the number of copies of this sequence in the human genome is 26 ± 2. We have sequenced and analyzed 19 genomic clones corresponding to the 37LRP/p40 gene and found that they were all processed pseudogenes. They all lack intronic sequences and show multiple genetic alterations leading in some cases to the appearance of stop codons. Moreover, they all bear characteristic features of retroposons as the presence of a poly(A)-tail at their 3′ end and short direct repeated flanking DNA sequences. None of the pseudogenes analyzed present cis-elements in their 5′ flanking region such as TATA or GC boxes. Our data reveal that over 50% of the 37LRP/p40 gene copies are pseudogenes most probably generated by retropositional events. The finding of multiple pseudogenes for the 37LRP/p40 gene suggests that the accumulation of several copies of this gene might have given a survival advantage to the cell in the course of evolution.     

67-kDa laminin receptor promotes internalization of cytotoxic necrotizing factor 1-expressing Escherichia coli K1 into human brain microvascular endothelial cells

Escherichia coli K1 is the most common Gram-negative organism causing meningitis, and its invasion of human brain microvascular endothelial cells (HBMEC) is a prerequisite for penetration into the central nervous system. We have reported previously that cytotoxic necrotizing factor 1 (CNF1) contributes to E. coli K1 invasion of HBMEC and interacts with 37-kDa laminin receptor precursor (37LRP) of HBMEC, which is a precursor of 67-kDa laminin receptor (67LR). In the present study, we examined the role of 67LR in the CNF1-expressing E. coli K1 invasion of HBMEC. Immunofluorescence microscopy and ligand overlay assays showed that 67LR is present on the HBMEC membrane and interacts with CNF1 protein as well as the CDPGYIGSR laminin peptide. 67LR was up-regulated and clustered at the sites of E. coli K1 on HBMEC in a CNF1-dependent manner. Pretreatment of CNF1+ E. coli K1 with recombinant 37-kDa laminin receptor precursor reduced the invasion rate to the level of Deltacnf1 mutant, and the invasion rate of CNF1+ E. coli K1 was enhanced in 67LR-overexpressing HBMEC, indicating 67LR is involved in the CNF1+ E. coli K1 invasion of HBMEC. Coimmunoprecipitation analysis showed that, upon incubation with CNF1+ E. coli K1 but not with Deltacnf1 mutant, focal adhesion kinase and paxillin were recruited and associated with 67LR. When immobilized onto polystyrene beads, CNF1 was sufficient to induce internalization of coupled beads into HBMEC through interaction with 67LR. Taken together, this is the first demonstration that E. coli K1 invasion of HBMEC occurs through the ligand-receptor (CNF1-67LR) interaction, and 67LR promotes CNF1-expressing E. coli K1 internalization of HBMEC.     

Invasion of tumorigenic HT1080 cells is impeded by blocking or downregulating the 37-kDa/67-kDa laminin receptor

The 37-kDa/67-kDa laminin receptor precursor/laminin receptor (LRP/LR) acting as a receptor for prions and viruses is overexpressed in various cancer cell lines, and their metastatic potential correlates with LRP/LR levels. We analyzed the tumorigenic fibrosarcoma cell line HT1080 regarding 37-kDa/67-kDa LRP/LR levels and its invasive potential. Compared to the less invasive embryonic fibroblast cell line NIH3T3, the tumorigenic HT1080 cells display approximately 1.6-fold higher cell-surface levels of LRP/LR. We show that anti-LRP/LR tools interfere with the invasive potential of HT1080 cells. Anti-LRP/LR single-chain variable fragment antibody (scFv) iS18 generated by chain shuffling from parental scFv S18 and its full-length version immunoglobulin G1-iS18 reduced the invasive potential of HT1080 cells significantly by 37% and 38%, respectively. HT1080 cells transfected with lentiviral plasmids expressing small interfering RNAs directed against LRP mRNA showed reduced LRP levels by approximately 44%, concomitant with a significant decrease in the invasive potential by approximately 37%. The polysulfated glycans HM2602 and pentosan polysulfate (SP-54), both capable of blocking LRP/LR, reduced the invasive potential by 20% and 35%, respectively. Adhesion of HT1080 cells to laminin-1 was significantly impeded by scFv iS18 and immunoglobulin G1-iS18 by 60% and 68%, respectively, and by SP-54 and HM2602 by 80%, suggesting that the reduced invasive capacity achieved by these tools is due to the perturbation of the LRP/LR-laminin interaction on the cell surface. Our in vitro data suggest that reagents directed against LRP/LR or LRP mRNA such as antibodies, polysulfated glycans, or small interfering RNAs, previously shown to encompass an anti-prion activity by blocking or downregulating the prion receptor LRP/LR, might also be potential cancer therapeutics blocking metastasis by interfering with the LRP/LR-laminin interaction in neoplastic tissues.     

Downregulation of the Non-Integrin Laminin Receptor Reduces Cellular Viability by Inducing Apoptosis in Lung and Cervical Cancer Cells

The non-integrin laminin receptor, here designated the 37-kDa/67-kDa laminin receptor (LRP/LR), is involved in many physiologically relevant processes, as well as numerous pathological conditions. The overexpression of LRP/LR on various cancerous cell lines plays critical roles in tumour metastasis and angiogenesis. This study investigated whether LRP/LR is implicated in the maintenance of cellular viability in lung and cervical cancer cell lines. Here we show a significant reduction in cellular viability in the aforementioned cell lines as a result of the siRNA-mediated downregulation of LRP. This reduction in cellular viability is due to increased apoptotic processes, reflected by the loss of nuclear integrity and the significant increase in the activity of caspase-3. These results indicate that LRP/LR is involved in the maintenance of cellular viability in tumorigenic lung and cervix uteri cells through the blockage of apoptosis. Knockdown of LRP/LR by siRNA might represent an alternative therapeutic strategy for the treatment of lung and cervical cancer.     

Scrapie-Infected Transgenic Mice Expressing a Laminin Receptor Decoy Mutant Reveal a Prolonged Incubation Time Associated with Low Levels of PrPres

The 37-kDa/67-kDa laminin receptor (LRP/LR) was identified as a cell surface receptor for prion proteins. The laminin receptor mutant LRP102-295∷FLAG interfered with PrP^Sc propagation in murine neuronal cells presumably acting as a decoy in a transdominant negative fashion by trapping PrP molecules in the extracellular matrix. Here, we generated hemizygous transgenic mice expressing LRP102-295∷FLAG in the brain. Scrapie-infected transgenic mice exhibit a significantly prolonged incubation time in comparison to scrapie-infected wild-type (FVB) mice. At the terminal stage, transgenic mice revealed significantly reduced proteinase-K-resistant PrP levels by 71% compared to wild-type mice. Our results recommend the laminin receptor decoy mutant as an alternative therapeutic tool for treatment of transmissible spongiform encephalopathies.     

Laminin Receptor 37/67LR Regulates Adhesion and Proliferation of Normal Human Intestinal Epithelial Cells

Interactions between the cell basal membrane domain and the basement membrane are involved in several cell functions including proliferation, migration and differentiation. Intestinal epithelial cells can interact with laminin, a major intestinal basement membrane glycoprotein, via several cell-surface laminin-binding proteins including integrin and non-integrin receptors. The 37/67kDa laminin receptor (37/67LR) is one of these but its role in normal epithelial cells is still unknown. The aim of this study was to characterise the expression pattern and determine the main function of 37/67LR in the normal human small intestinal epithelium. Immunolocalization studies revealed that 37/67LR was predominantly present in the undifferentiated/proliferative region of the human intestinal crypt in both the immature and adult intestine. Using a human intestinal epithelial crypt (HIEC) cell line as experimental model, we determined that 37/67LR was expressed in proliferative cells in both the cytoplasmic and membrane compartments. Small-interfering RNA-mediated reduction of 37/67LR expression led to HIEC cell-cycle reduction and loss of the ability to adhere to laminin-related peptides under conditions not altering ribosomal function. Taken together, these findings indicate that 37/67LR regulates proliferation and adhesion in normal intestinal epithelial cells independently of its known association with ribosomal function.     

Evidence for a precursor of the high-affinity metastasis-associated murine laminin receptor

The high-affinity cellular receptor for the basement membrane component laminin is differentially expressed during tumor invasion and metastasis. A cDNA clone encoding the murine laminin receptor was isolated and identified on the basis of sequence homology to the human laminin receptor [Wewer et al. (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 7137-7141]. Primer extension experiments demonstrated that the clone contained the complete 5' sequence of the murine laminin receptor mRNA. RNA blot data demonstrated a single-sized laminin receptor mRNA, approximately 1400 bases long, in human, mouse, and rat. The nascent laminin receptor predicted from the cDNA sequence is 295 amino acids long, with a molecular weight of 33,000, and contains one intradisulfide bridge, a short putative transmembrane domain, and an extracellular carboxy-terminal region which has abundant glutamic acid residues and multiple repeat sequences. The precursor of the laminin receptor is apparently smaller than the 67-kilodalton protein isolated from tissue. The apparent molecular weight on SDS-polyacrylamide gels of the rabbit reticulocyte cell-free translation product of selectively hybridized laminin receptor mRNA is 37,000. Antisera to three different domains of the cDNA-predicted receptor were used to study the relationship between the 37- and 67-kilodalton polypeptides. Antisera to cDNA-deduced synthetic peptides of the receptor immunoprecipitated a 37-kilodalton band both from cell-free translation products and from pulse-labeled cell extracts. On immunoblots of cell extracts, one antisynthetic peptide antiserum recognized only the 67-kilodalton receptor, while another antiserum identified both 37- and 67-kilodalton polypeptides, suggesting a precursor-product relationship between the two polypeptides.