The protein complexity of the cytoskeleton of bovine and human sperm heads: the identification and characterization of cylicin II.

The cytoskeletal calyx of mammalian sperm heads surrounding the basolateral part of the nucleus contains two kinds of basic proteins: Calicin, a polypeptide of approximate M(r) 60,000 as estimated from SDS-PAGE, and the group of the very basic cylicins (pI > 10.0), formerly designated as "multiple-band polypeptides." Recently, bovine cylicin I has been cDNA cloned and identified as a new type of a cytoskeletal protein, which contains numerous Lys-Lys-Asp tripeptides accumulated in nine central repetitive units predicted to form alpha-helices. We now report the cDNA cloning and localization of a second species of cylicin, bovine cylicin II, present in bovine and human sperm heads: Cylicin II (488 amino acids, M(r) 53,561, pI 10.55) shows the same prominent molecular characteristics as cylicin I, including a high content of charged amino acids, the abundance of Lys-Lys-Asp tripeptides, and repetitive units of presumably alpha-helical configuration, but also presents some differences. As with cylicin I mRNA, the 2.6-kb mRNA has also been shown to be specifically expressed in testis. The possible existence of a larger cylicin multigene family and its contribution to the cytoskeleton and the morphogenesis of the sperm head are discussed.