MOLECULAR CHARACTERIZATION OF MAMMALIAN CYLICIN, A BASIC-PROTEIN OF THE SPERM HEAD CYTOSKELETON

The cytoskeletal calyx structure surrounding part of the nucleus of th e mammalian sperm head contains two major kinds of basic proteins, i.e ., the approximately 60-kD calicin and a group of very basic (IEP > pH 10) polypeptides ranging in size from approximately 58 to approximate ly 100 kD (''multiple band proteins,'' MBPs). We have produced MBP-spe cific mAbs and have isolated a bovine and a human cDNA clone encoding one of these proteins, termed ''cylicin'' (from the Greek word eta(c) kappaupsilonlambdaiotaxi for cup or beaker). Bovine cylicin I of a cal culated molecular weight of 74,788 contains a high proportion (29%) of positively charged amino acids, resulting in an IEP of 10.55, numerou s KKD tripeptides, and is characterized by an organization of the cent ral part of the molecule in nine repeating units of maximally 41 amino acids each of which according to prediction analysis should tend to f orm an alpha helix. The identity of the polypeptide has been proven by direct amino acid sequencing of > 14 different fragments and by exper iments using antibodies raised against a partial cDNA-derived protein segment produced in E. coli. By Northern blot analysis we have identif ied the 2.4-kb cylicin I mRNA only in testis. The unusual cytoskeletal protein cylicin is compared with other proteins and its possible arch itectural role during spermiogenesis is discussed.