Sequence of a human transcript expressed in T-lymphocytes and encoding a fibrinogen-like protein

A murine cDNA sequence (termed pT49), encoding a protein related to the fibrinogen (Fib) beta and gamma chains, has been previously reported [Koyama et al., Proc. Natl. Acad. Sci. USA 84 (1987) 1609-1613]. We used a murine pT49 probe to screen a human small intestine cDNA library, and obtained a set of overlapping cDNA clones encoding a Fib-like protein that is 80% identical with the product of the murine pT49 gene. The deduced amino acid (aa) sequence contains a predicted signal peptide and five consensus motifs for N-linked glycosylation. The presence of conserved Cys residues involved in the assembly of the mature Fib complex and of two alpha-helical regions permissive for coiled-coil formation, suggests that this Fib-like protein may be secreted as a multichain complex. Two mRNA species of approx. 4.5 and approx. 1.5 kb were detected by Northern blot hybridization of a human pT49-homolog cDNA probe with RNA obtained from resting peripheral blood T-lymphocytes. By RT-PCR analysis of purified peripheral blood T-lymphocyte subsets, we found expression of the pT49-homolog transcript in both CD3+/CD4+ and CD3+/CD8+ T-lymphocytes. The coding region of the human pT49-homolog cDNA was fused at its 3' end with a tag-coding sequence and was expressed in CHO cells. The corresponding gene product was immunoprecipitated with an anti-tag antibody from the cell lysate and from the culture supernatant of metabolically labeled transfectants and was identified as approx. 62 and approx. 64-70-kDa proteins, respectively.