The extra segments of sequence in rat leucocyte common antigen (L-CA) are derived by alternative splicing of only three exons and show extensive O-linked glycosylation.

The leucocyte common antigen (L-CA, CD45, or T200) consists of a family of heavily glycosylated glycoproteins of apparent Mr 180,000-240,000 found at the surface of leucocytes but not other cell types. Populations of lymphocytes express forms that differ in antigenicity, apparent Mr, and glycosylation. Some of this heterogeneity is due to polypeptide differences caused by the insertion of up to three different segments of sequence near the NH2-terminus. We report the complete sequence of the region of the rat L-CA gene encoding the extra segments. Analysis of this sequence showed that each segment was encoded by an exon but no further exons could be identified, implying that the polypeptide heterogeneity is solely due to selection from these three exons by alternative splicing. Amino acid sequencing of glycopeptides prepared from the largest forms of L-CA indicated extensive O-linked glycosylation in at least one of the extra segments.