Three soluble form messages of murine CD46 are produced through alternative mRNA splicing

Murine CD46 (mCD46) is a type 1 membrane protein expressed predominantly in testicular germ cells, the distribution profile of which is in contrast to that of human CD46 showing a ubiquitous tissue distribution. We have identified an additional message of mCD46 that encodes a putative secretory form [Nomura et al. (1999) Immunogenetics 50, 245-254]. Here, we cloned three cDNAs encoding putative soluble CD46 from murine testis. These soluble form messages were yielded on insertion of unidentified nucleotide sequences, 77, 179, and 73 ntds, into the junctions between the SCR3 and SCR4 (variant 2), ST(c) and UK (variant 3), and SCR4 and ST(c) (variant 1) domains, respectively, the last one corresponding to the reported soluble form. The exons corresponding to these three inserts were identified in the murine CD46 genome, indicating that the alternative splicing of mRNA participates in the generation of these various CD46 messages. In normal mouse sera and cell lines, however, virtually no soluble CD46 was detected on immunoblotting. On Northern blotting analysis with specific probes, on the other hand, variant 1 was found to be predominantly expressed in the liver and heart. In addition, all variant messages were detected on PCR in all organs examined. When a rabbit cell line, RK13 cells, was transfected with cDNA of variant 1, protein synthesis was detected on immunoblotting. Although the mCD46 protein production was inefficient, this variant 1 exhibited factor I-cofactor activity as to inhibition of the complement cascade. Since the mCD46 protein was reported to be markedly up-regulated on infection of murine cells with mCMV, the soluble mCD46 proteins may act as a complement regulator that controls the systemic complement system under the conditions of a viral infection.