Identification of structural elements of the testis-specific voltage dependent calcium channel that potentially regulate its biophysical properties

Calcium influx through voltage-dependent calcium channels regulates the phy siological acrosome reaction of mammalian spermatozoa. Expression of the mR NA for these voltage-dependent calcium channels and its co-ordinated transl ation is initiated early in rat mate germ line development and continues th roughout spermatogenesis. Herein, we report the complete mRNA and deduced a mino acid sequence of the alpha 1(C) pore-forming subunit of the rat testis -specific L-type calcium channel. This subunit is transcribed from the alph a 1(C) gene, which is also expressed in brain and cardiac muscle. The cardi ac- and testis-specific isoforms of the ale subunit are produced by alterna te splicing of the same primary transcript. The testis-specific isoform dif fers from that of cardiac tissue at its amino terminus and in transmembrane segments IS6, IIIS2 and IVS3, which are also dihydropyridine binding sites . In somatic tissues, segments S2 and S3 regulate channel activation while the amino terminus and segment IS6 contribute to channel inactivation kinet ics. The amino terminus and IS6 segment of the testis-specific alpha 1(C) s ubunit are also expressed respectively, in the brain and in smooth muscle f rom lung where they alter the electrophysiological characteristics of the s ubunit to produce relatively slow inactivation kinetics. These findings pro vide a molecular explanation for the detection by others, by patch clamp an alysis, of T-type calcium currents in immature spermatogenic cells and of a typical L-type calcium currents in mature spermatozoa.