A prolactin-inducible gene product which is a member of the calpactin/lipocortin family.

The major PRL-inducible gene product from pigeon cropsac was cloned from lambda gt11 and sequenced by chain termination. The sequence of pGcp35 includes an open reading frame which yields a polypeptide which highly similar to mammalian calpactin II (lipocortin I). Like the other calpactins, the deduced protein (cp35) consists of a 4-fold repeating structure which has a conserved core characteristic of a large group of calcium-dependent membrane-binding proteins. PRL-stimulated cropsac expresses a calcium-dependent membrane-binding protein which is the proper size for endogenous cp35. Detailed comparison of the sequences of cp35 and human calpactin II shows that the only substantial sequence dissimilarity is a domain encoding amino acids between residues 20 and 40 which includes a tyrosine phosphorylation site in the human molecule, along with other residues of possible physiological significance. These results raise the possibility that calpactins are regulated by PRL in other tissues; and, that the sequences of the avian form and the mammalian form may have selectively diverged to yield different regulatory mechanisms.