Molecular nature of calicin, a major basic protein of the mammalian sperm head cytoskeleton

In the mammalian sperm head the nucleus is tightly associated, in many species in its posterior part, with a large and dense nonfilamentous cytoskeletal structure, the calyx, whose major proteins are basic, representing a novel category of cytoskeletal element. Using specific antibodies, biochemical methods, and cDNA cloning we have characterized one of these calyx proteins, previously termed calicin, in bull and man. The polypeptide of 588 amino acids (Mr of 66,889; IEP 8.1) is very similar in the two species and is encoded by a approximately 2.2-kb mRNA that has been detected only in testis but not in any other tissue or cell culture examined. Sequence analysis has revealed that calicin is homologous to the kelch protein of the ring canal structure of Drosophila ovaries. In particular, it contains three consecutive repeating units of 48 amino acids each which are homologous to the so-called "beta-strand folds" occurring in proteins of the kelch family, including the actin cross-linking protein scruin of Limulus sperm and a series of other eukaryotic, bacterial, and viral proteins. Moreover, the amino terminal domain of calicin contains a region of about 100 amino acids homologous to an extended motif shared by the kelch protein as well as various zinc finger and poxvirus proteins. The possible role of calicin as a morphogenic cytoskeletal element in spermiogenic differentiation is discussed, also in relation to the demonstrated absence or altered arrangement of calicin in frequent forms of human teratozoospermia such as "round-headed" or other "postacrosomal sheath defect" sperm malformations.