p36 is a major substrate of both viral and growth factor receptor associated protein kinases. This protein has recently been named calpactin I heavy chain since it is the large subunit of a Ca2+-dependent phospholipid and actin binding heterotetramer. The primary structure of p36 has been determined from analysis of cloned cDNA. The protein contains 338 amino acids, has an approximate molecular weight of 39000, and is comprised of several distinct domains, including four 75 amino acid repeats. From two overlapping cosmid clones isolated from different mouse genomic liver libraries, the complete intron/exon structure of the p36 gene was determined and the 5′ and 3′ noncoding regions of the gene were analyzed. The coding and 3′ untranslated region of the p36 gene contains 12 exons which range in size from 48 to 322 base pairs (bp) with an average size of 107 bp. The repeat structures found at the protein level are not delineated by single exons, but the N-terminal p11-binding domain is encoded by a single exon. Structural mapping of the gene demonstrated that the lengths of the first two introns in the coding region are together approximately 6 kilobases (kb), while the other introns range in size from 600 to 3600 bp with an average size of 1650 bp. The p36 gene is at least 22 kb in length and has a coding sequence of approximately 1 kb, representing only 4.5% of the gene. Analysis of mouse X Chinese hamster hybrid cell lines, together with typing of a restriction fragment length polymorphism in recombinant inbred strains of mice, localized the p36 gene (Callh) to chromosome 9, midway between Apoal and d. © 1990, American Chemical Society. All rights reserved.