Molecular and functional characterization of mouse S5D-SRCRB: a new group B member of the scavenger receptor cysteine-rich superfamily.

The scavenger receptor cysteine-rich superfamily (SRCR-SF) members are transmembrane and/or secreted receptors exhibiting one or several repeats of a cysteine-rich protein module of ∼100 aa, named scavenger receptor cysteine-rich (SRCR). Two types of SRCR domains (A or B) have been reported, which differ in the number of coding exons and intradomain cysteines. Although no unifying function has been reported for SRCR-SF members, recognition of pathogen-associated molecular patterns (PAMPs) was recently shown for some of them. In this article, we report the structural and functional characterization of mouse S5D-SRCRB, a new group B member of the SRCR-SF. The s5d-srcrb gene maps at mouse chromosome 7 and encompasses 14 exons extending over 15 kb. The longest cDNA sequence found is 4286 bp in length and encodes a mature protein of 1371 aa, with a predicted M(r) of 144.6 kDa. Using an episomal mammalian-expression system, a glycosylated soluble recombinant form >200 kDa was obtained and used as immunogen for the generation of specific rat mAbs. Subsequent immunohistochemical and real-time PCR analysis showed significant S5D-SRCRB expression in murine genitourinary and digestive tracts. S5D-SRCRB was shown to bind endogenous extracellular matrix proteins (laminin and galectin-1), as well as PAMPs present on Gram-positive and Gram-negative bacteria and fungi. PAMP binding by S5D-SRCRB induced microbial aggregation and subsequent inhibition of PAMP-induced cytokine release. These abilities suggest that S5D-SRCRB might play a role in the innate defense and homeostasis of certain specialized epithelial surfaces.

Crystal structure of the third extracellular domain of CD5 reveals the fold of a group B scavenger cysteine-rich receptor domain.

Scavenger receptor cysteine-rich (SRCR) domains are ancient protein modules widely found among cell surface and secreted proteins of the innate and adaptive immune system, where they mediate ligand binding. We have solved the crystal structure at 2.2 A of resolution of the SRCR CD5 domain III, a human lymphocyte receptor involved in the modulation of antigen specific receptor-mediated T cell activation and differentiation signals. The first structure of a member of a group B SRCR domain reveals the fold of this ancient protein module into a central core formed by two antiparallel beta-sheets and one alpha-helix, illustrating the conserved core at the protein level of genes coding for group A and B members of the SRCR superfamily. The novel SRCR group B structure permits the interpretation of site-directed mutagenesis data on the binding of activated leukocyte cell adhesion molecule (ALCAM/CD166) binding to CD6, a closely related lymphocyte receptor homologue to CD5.

The Scavenger Receptor Cysteine-Rich (SRCR) domain: an ancient and highly conserved protein module of the innate immune system.

The Scavenger Receptor Cysteine-Rich (SRCR) domain is an ancient and highly conserved protein module of ~100-110 amino acids, which defines a superfamily (SRCR-SF) of either soluble or membrane-bound receptors expressed by hematopoietic and nonhematopoietic cells, at either embryonic or adult stages. The existence of two types of SRCR domains allows the division of the SRCR-SF into two groups. Members of group A contain SRCR domains with 6 cysteine residues and are encoded by two exons, whereas those of group B usually contain 8 cysteines and are encoded by a single exon. Group A members usually present as multidomain mosaic proteins containing single SRCR domains associated to other functional domains, such as enzymatic (protease) domains or collagenous regions. On the contrary, group B members generally present as proteins exclusively composed of tandem repeats of SRCR domains, with or without the presence of CUB and ZP domains thought to be involved in oligomerization but never associated to protease domains. Representatives of either group are found in different animal species, from low invertebrates (sponges) to high vertebrates (mammals). Although no unifying function has been defined for SRCR-SF members, accumulated data, together with the high degree of structural and phylogenetic conservation of SRCR domains indicates that they might subserve basic homeostatic functions, including innate immune defense.

Cloning of S4D-SRCRB, a new soluble member of the group B scavenger receptor cysteine-rich family (SRCR-SF) mapping to human chromosome 7q11.23.

The scavenger receptor cysteine-rich superfamily (SRCR-SF) is a highly conserved group of membrane and/or secreted proteins related to the innate and adaptive immune system. Here, we report the cloning of the gene encoding human S4D-SRCRB, a novel soluble member of the SRCR-SF, which is composed of four group B SRCR domains separated by Pro-, Ser- and Thr-rich polypeptides. The longest cDNA sequence found is 2,806 bp in length and encodes a mature protein of 528 aa, with a predicted molecular mass of M(r) 55,600. The S4D-SRCRB gene is located at Chromosome 7q11.23, telomeric to the Williams-Beuren syndrome deletion. It extends over 20 kb and consists of 11 exons, with each SRCR domain being encoded by a single exon. Northern blot analysis indicated that S4D-SRCRB has a broad tissue distribution and is expressed as two major mRNA species: one of 2.8 kb, with a restricted tissue expression pattern (mainly kidney and placenta), and another of 1.5 kb, with a broader distribution. A similar mRNA expression pattern was observed during the analysis of several tumor cell lines. The highest degree of similarity found between S4D-SRCRB and other group B SRCR-SF members was with human DMBT1 (a mosaic protein composed of fourteen SRCR domains, which is involved in innate defense and epithelia polarization) and chicken 18-B (a turpentine-induced soluble acute-phase protein composed of four SRCR domains). Our data indicate that S4D-SRCRB constitutes a novel SRCR-SF member, which could be involved in basic homeostatic functions such as innate host defense.