Tissue‐specific and allelic expression of the complement regulator CD46 is controlled by alternative splicing

Abstract

CD46 (membrane cofactor protein) is a human cell surface glycoprotein with cofactor activity for factor I-mediated cleavage of complement components C3b and C4b. The CD46 protein from normal lymphocytes resolves on sodium dodecyl sulfate-polyacrylamide gel electrophoresis as two major bands of 66 and 56 kDa. CD46 cDNA encodes four extracellular short consensus repeat domains, a Ser/Thr/Pro (STP)-rich region, a transmembrane region and a cytoplasmic tail. We now show that exquisite control of mRNA splicing is responsible for the heterogeneous expression of CD46 isoforms. Differential splicing of 5 exons generates at least 14 CD46 mRNA variants whose expression is stringently regulated by allelic, tissue-specific and malignancy-related factors, as: (a) leukemic cells and Epstein-Barr virus-transformed B cells preferentially incorporate the first of three STP exons (exon 7) into mRNA, and produce a larger CD46 isoform of 74 kDa, (b) an allelic difference in the proportion of 66- and 56-kDa CD46 isoforms on lymphocytes corresponds to the preferential inclusion or exclusion of the second STP exon (exon 8), (c) the third STP exon (exon 9) is specifically deleted in some placentae, (d) spermatozoa delete both exons 12 and 13, encoding a shorter transmembrane region and a unique cytoplasmic tail and (e) all tissues tested differentially splice exon 13, resulting in two alternative cytoplasmic tails. The distribution of the 14 alternatively spliced RNA transcripts correlated with the presence of protein isoforms of the predicted size, indicating that alternative splicing leads to heterogeneity of CD46 glycoproteins.

A CD46-like Molecule Functional in Teleost Fish Represents an Ancestral Form of Membrane-Bound Regulators of Complement Activation

“…We have also cloned its carp homolog for further phylogenetic and functional studies and revealed that cTecrem is closely similar to zTecrem, but differs in the domain structure such as number of SCR modules (4 in carp, 5 in zebrafish). The diversity in the number of SCR modules is common in the member of RCA family (45,46), and a minimum functional unit as a binding site usually spans only a single or two SCR modules (47)(48)(49)(50)(51)(52)(53)(54)(55). Thus, it seems unlikely that the difference in the number of SCR modules would significantly affect functions of Tecrem between the two species.…”

“…ference seen on the threonine/proline-rich region may have a functional effect on the Tecrem molecule, because in mammalian MCP/CD46, O-glycosylation in the STP region controls its protein expression and complement-regulatory function (44,55). In this regard, it should be noted that these splicing variants contained different length and potential O-glycosylation sites of threonine/ proline-rich regions and may cause functional divergence of Tecrem.…”

Identification and characterization of a silencer regulatory element in the 3′-flanking region of the murine CD46 gene

“…Rodents adopt this for species identification in fertilization, whereas primates use it for self-protection from homologous C in the immune system. The murine CD46 has only one Ser\Thr-rich (ST) domain, ST C , similar to the human testicular isoform, whereas the ubiquitous human isoforms have combinations of the three ST domains, ST A , ST B and ST C [48,49]. Thus both human and mouse testicular germ cells\spermatozoa constitutively express only the ST C isoform of CD46 [16,17].…”

Cutting Edge: Inhibiting Measles Virus Infection but Promoting Reproduction: An Explanation for Splicing and Tissue-Specific Expression of CD46

“…There are three exons in the STP region whose translated products are referred to as A, B, and C. The C terminus of MCP consists of one of two possible cytoplasmic tails referred to as CYT-1 or CYT-2. MCP is expressed as isoforms that arise via alternative splicing of the STP and cytoplasmic tail regions (28,29). I, Common human isoforms.…”