cDNA sequence of a novel sex-limited protein (Slp) from mice constitutive for Slp expression. Sequence comparisons suggest that Slp has no functional role.

Murine sex-limited protein (Slp) is a serum protein that shares 95% sequence identity with murine complement component C4 but does not have C4 activity. Mouse strain B10.WR, which carries the H-2w7 haplotype, has up to 4 Slp genes and is unusual in that both males and females express Slp. Here we report the sequence of a complete pro-Slp cDNA from this strain that we designate Slpw7.2. We find that the Slpw7.2 sequence differs at multiple dispersed sites from three previously reported Slp sequences: two complete pro-Slp sequences, Slpw7.1 and SlpFM, from the B10.WR and FM strains, respectively, and a partial sequence from the B10.WR strain that is distinct from Slpw7.1 as well. A detailed comparison of the complete Slpw7.1, Slpw7.2, and SlpFM sequences reveals that nucleotide changes that alter the amino acid sequence (replacement substitutions) are accumulating at the same relative rate as changes that do not affect the amino acid sequence (silent substitutions); in addition, the amino acid changes themselves tend to be nonconservative. Our results suggest that at least three Slp genes are transcriptionally active in B10.WR mice; that the protein product of the Slpw7.2 transcript predominates in B10.WR serum; and that the Slp protein probably has no function. The Slp system may be particularly suitable for the study of the evolution in the absence of selective pressures of a gene that encodes a stable protein.