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

Looking into laminin receptor: critical discussion regarding the non-integrin 37/67-kDa laminin receptor/RPSA protein

The 37/67-kDa laminin receptor (LAMR/RPSA) was originally identified as a 67-kDa binding protein for laminin, an extracellular matrix glycoprotein that provides cellular adhesion to the basement membrane. LAMR has evolutionary origins, however, as a 37-kDa RPS2 family ribosomal component. Expressed in all domains of life, RPS2 proteins have been shown to have remarkably diverse physiological roles that vary across species. Contributing to laminin binding, ribosome biogenesis, cytoskeletal organization, and nuclear functions, this protein governs critical cellular processes including growth, survival, migration, protein synthesis, development, and differentiation. Unsurprisingly given its purview, LAMR has been associated with metastatic cancer, neurodegenerative disease and developmental abnormalities. Functioning in a receptor capacity, this protein also confers susceptibility to bacterial and viral infection. LAMR is clearly a molecule of consequence in human disease, directly mediating pathological events that make it a prime target for therapeutic interventions. Despite decades of research, there are still a large number of open questions regarding the cellular biology of LAMR, the nature of its ability to bind laminin, the function of its intrinsically disordered C-terminal region and its conversion from 37 to 67 kDa. This review attempts to convey an in-depth description of the complexity surrounding this multifaceted protein across functional, structural and pathological aspects.

Molecular characterization of the full-length coding sequence of the caprine laminin receptor gene (RPSA)

Scrapie is a prion disease in sheep and goats. Ribosomal protein SA (RPSA), also called 37 kDa laminin receptor precursor/67 kDa laminin receptor has been demonstrated to be a putative cell surface receptor for prion. To investigate the caprine RPSA, we cloned the full-length coding sequence of the gene of goat and submitted it to GenBank. The length of the open reading frame is 888 bp, encoding 295 amino acids. The putative amino acid sequence is highly similar to that of other mammals. The caprine amino acid sequence of RPSA is shown to be identical to the sequence of species susceptible to scrapie at positions 241, 272, and 291. The phylogenetic tree analysis revealed that the genetic distance between sheep and goat is the smallest. Moreover, RT-PCR results of 11 tissues indicated that RPSA mRNA is expressed in all selected caprine tissues.

Structural and functional analysis of the ovine laminin receptor gene (RPSA): Possible involvement of the LRP/LR protein in scrapie response

Scrapie is a prion disease affecting sheep and goats. Susceptibility to this neurodegenerative disease shows polygenic variance. The involvement of the laminin receptor (LRP/LR) in the metabolism and propagation of prions has previously been demonstrated. In the present work, the ovine laminin receptor gene (RPSA) was isolated, characterized, and mapped to ovine chromosome OAR19q13. Real-time RT-PCR revealed a significant decrease in RPSA mRNA in cerebellum after scrapie infection. Conversely, no differences were detected in other brain regions such as diencephalon and medulla oblongata. Association analysis showed that a polymorphism reflecting the presence of a RPSA pseudogene was overrepresented in a group of sheep resistant to scrapie infection. No amino acid change in the LRP/LR protein was found in the 126 sheep analyzed. However, interesting amino acid positions (241, 272, and 290), which could participate in the species barrier to scrapie and maybe to other transmissible spongiform encephalopathies, were identified by comparing LRP/LR sequences from various mammals with variable levels of resistance to scrapie.

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.

Novel Aspects of Prions, Their Receptor Molecules, and Innovative Approaches for TSE Therapy

1. Prion diseases are a group of rare, fatal neurodegenerative diseases, also known as transmissible spongiform encephalopathies (TSEs), that affect both animals and humans and include bovine spongiform encephalopathy (BSE) in cattle, scrapie in sheep, chronic wasting disease (CWD) in deer and elk, and Creutzfeldt-Jakob disease (CJD) in humans. TSEs are usually rapidly progressive and clinical symptoms comprise dementia and loss of movement coordination due to the accumulation of an abnormal isoform (PrP(Sc)) of the host-encoded prion protein (PrP(c)). 2. This article reviews the current knowledge on PrP(c) and PrP(Sc), prion replication mechanisms, interaction partners of prions, and their cell surface receptors. Several strategies, summarized in this article, have been investigated for an effective antiprion treatment including development of a vaccination therapy and screening for potent chemical compounds. Currently, no effective treatment for prion diseases is available. 3. The identification of the 37 kDa/67 kDa laminin receptor (LRP/LR) and heparan sulfate as cell surface receptors for prions, however, opens new avenues for the development of alternative TSE therapies.

The Nonintegrin Laminin Binding Protein (p67 LBP) Is Expressed on a Subset of Activated Human T Lymphocytes and, Together with the Integrin Very Late Activation Antigen-6, Mediates Avid Cellular Adherence to Laminin

A search for genes expressed in activated T cells revealed that the nonintegrin, 67-kDa laminin binding protein (p67 LBP) is expressed on the surface of a subset (10–15%) of activated peripheral blood T cells. Surface p67 LBP expression is detectable by FACS using the anti-p67 LBP mAb, MLuC5, within 6 h of T cell activation with phorbol dibutyrate and ionomycin, peaks 18–36 h postactivation, and persists for 7–10 days. The subset of T cells expressing p67 LBP is composed of mature, single-positive cells (85% CD4+8−, 15% CD4−8+) of memory cell phenotype (100% CD45 RO+/CD45 RA−). The p67 LBP+ T cells also express the integrin α6 chain (CD49f), which is known to associate with p67 LBP on tumor cells. In addition, the p67 LBP+ T cells express the integrin β1, which associates with α6 in the laminin-specific integrin receptor very late activation Ag (VLA)-6 (α6β1). Expression of an exogenous cDNA encoding the 37-kDa LBP precursor (p37 LBPP) confers p67 LBP surface expression on a p67 LBP-negative Jurkat T cell line (B2.7). Expression of p67 LBP induces B2.7 transfectants to adhere to laminin, but avid laminin binding depends on coexpression of VLA-6. Taken together, these data indicate that p67 LBP is an activation-induced surface structure on memory T cells that, together with VLA-6, mediates cellular adherence to laminin.

Role of the 37 kDa laminin receptor precursor in the life cycle of prions

Prions are thought to consist of infectious proteins that cause, in the absence of detectable nucleic acid, a group of fatal neurodegenerative diseases, called transmissible spongiform encephalopathies (TSE). Among these diseases are bovine spongiform encephalopathy (BSE), scrapie of sheep and Creutzfeldt-Jakob disease (CJD) in humans. They occur as sporadic, infectious or genetic disorders and have in common the accumulation of an abnormal, pathogenic isoform of the cellular prion protein PrPc which is converted in a post-translational process into PrPSc concomitant with conformational changes of the protein. During this process PrPc acquires a high beta-sheet content and becomes partially resistant to proteases. The mechanism of this conversion as well as the physiological function of the cellular prion protein PrPc are poorly understood, but studies employing PrP knock-out mice demonstrated that PrPc is required for the development of prion diseases. The involvement of co-factors such as chaperones, receptors or an unknown protein, designated "protein X" in the conversion process are discussed. In a yeast two-hybrid screen we have identified the 37 kDa laminin receptor precursor (LRP) as an interactor of the cellular prion protein and this interaction could be confirmed by co-infection and co-transfection studies in mammalian and insect cells. LRP evolved from the ribosomal protein p40 essential for protein synthesis lacking any laminin binding activity to a cell surface receptor binding laminin, elastin and carbohydrates. The gene encoding 37 kDa LRP/p40 has been identified in a variety of species including the sea urchin Urechis caupo, Chlorohydra viridissima, the archaebacterium Haloarcula marismortui, the yeast Saccharomyces cerevisiae as well as in mammals where it is highly conserved. LRP works as a receptor for alphaviruses and is associated with the metastatic potential of solid tumors where it was first identified. The 37 kDa LRP forms its mature 67 kDa isoform with high laminin binding capacity by an unknown mechanism involving acylation. The multifunctionality of LRP as a ribosomal protein and a cell surface receptor for infectious agents such as viruses and prions might be extended by additional properties.

The human 37-kDa laminin receptor precursor interacts with the prion protein in eukaryotic cells

Prions are thought to consist of infectious proteins that cause transmissible spongiform encephalopathies. According to overwhelming evidence, the pathogenic prion protein PrPSc converts its host encoded isoform PrPC into insoluble aggregates of PrPSc, concomitant with pathological modifications (for review, see refs. 1-3). Although the physiological role of PrPC is poorly understood, studies with PrP knockout mice demonstrated that PrPC is required for the development of prion diseases. Using the yeast two-hybrid technology in Saccharomyces cerevisiae, we identified the 37-kDa laminin receptor precursor (LRP) as interacting with the cellular prion protein PrPC. Mapping analysis of the LRP-PrP interaction site in S. cerevisiae revealed that PrP and laminin share the same binding domain (amino acids 161 to 180) on LRP. The LRP-PrP interaction was confirmed in vivo in insect (Sf9) and mammalian cells (COS-7). The LRP level was increased in scrapie-infected murine N2a cells and in brain and spleen of scrapie-infected mice. In contrast, the LRP concentration was not significantly altered in these organs from mice infected with the bovine spongiform encephalopathic agent (BSE), which have a lower PrPSc accumulation. LRP levels, however, were dramatically increased in brain and pancreas, slightly increased in the spleen and not altered in the liver of crapie-infected hamsters. These data show that enhanced LRP concentrations are correlated with PrPSc accumulation in organs from mice and hamsters. The laminin receptor precursor, which is highly conserved among mammals and is located on the cell surface, may act as a receptor or co-receptor for the prion protein on mammalian cells.