The N terminus of the cardiac L-type Ca2+ channel α1C subunit -: The initial segment is ubiquitous and crucial for protein kinase C modulation, but is not directly phosphorylated

The first 46 amino acids (aa) of the N terminus of the rabbit heart (RH) L-type cardiac Ca2+ channel alpha(1C) subunit are crucial for the stimulating action of protein kinase C (PKC) and also hinder channel gating (Shistik, E,, Ivanina, T,, Blumenstein, Y., and Dascal, N, (1998) J, Biol Chem, 273, 17901-17909); The mechanism of PKC action and the location of the PKC target site are not known. Moreover, uncertainties in the genomic sequence of the N-terminal region of alpha(1C) leave open the question of the presence of RH-type N terminus in L-type channels in mammalian tissues. Here, we demonstrate the presence of alpha(1C) protein containing an RH-type initial N-terminal segment in rat heart and brain by using a newly prepared polyclonal antibody. Using deletion mutants of alpha(1C) expressed in Xenopus oocytes, we further narrowed down the part of the N terminus crucial for both inhibitory gating and for PKC effect to the first 20 amino acid residues, and we identify the first 5 aa as an important determinant of PKC action and of N-terminal effect on gating, The absence of serines and threonines in the first 5 aa and the absence of phosphorylation by PKC of a glutathione S-transferase-fusion protein containing the initial segment suggest that the effect of PKC does not arise through a direct phosphorylation of this segment. We propose that PKC acts by attenuating the inhibitory action of the N terminus via phosphorylation of a remote site, in the channel or in an auxiliary protein, that interacts with the initial segment of the N terminus.