Résumé
The primary structure of a calcium channel beta subunit ( beta sub(M)) from housefly (Musca domestica) has been deduced by cDNA cloning and sequence analysis. The open reading frame encodes a 441-amino acid polypeptide with a calculated molecular mass of 48,755 Da. Whole-mount in situ hybridization indicates that beta sub(M) mRNA is predominantly expressed in neuronal tissues. Transcription of beta sub(M) mRNA is evident from stage 13/14 of embryogenesis up to adulthood. Different expression patterns of splice variants were found in larvae and in adult fly heads. Amino acid identity between beta sub(M) and mammalian beta subunits is lower (66-68%) than within mammalian beta subunits (74-80%). Calculation of a phylogenetic tree indicates that beta sub(M) is an ancestral form of the four distinct beta subunit gene products identified in mammalian tissues so far. Despite these sequence differences, beta sub(M) is able to enhance endogenous calcium channel activity in Xenopus laevis oocytes as well as dihydropyridine binding to membranes from COS 7 cells transfected with rabbit heart alpha sub(1) cDNA in the same manner as was previously shown for mammalian beta subunits.