Differential phosphorylation of two size forms of the neuronal class C L-type calcium channel α1 subunit

L-type calcium channels mediate long-lasting calcium currents which are modulated by protein phosphorylation. Using site-directed anti-peptide antibodies, we show that the al subunit of the neuronal class C L-type calcium channel from rat brain exists in two size forms. The longer form, L_C2, with an apparent molecular mass of 210-235 kDa was phosphorylated in vitro by cAMP-dependent'protein kinase (cA-PK), but the shorter form, L_C1, with an apparent molecular mass of 190-195 kDa was not a substrate for cA-PK. In contrast, L_C1 and L_C2 are both substrates for protein kinase C (PKC), calcium- and calmodulin-dependent protein kinase II, and cGMP-dependent protein kinase (cG-PK). The site-directed anti-peptide antibody CNC2 was produced against the COOH-terminal end of the class C L-type α1 subunit as predicted by molecular cloning and sequencing of cDNA. CNC2 recognized L_C2 but not L_C1 by immunoblotting and immunoprecipitated only L_C2 phosphorylated by either cA-PK or PKC. These results indicate that LC1 is truncated at its COOH-terminal end with respect to L_C2 and that cA-PK preferentially phosphorylates sites in the COOH-terminal region of the al subunit that are present in L_c2 but not L_C1. The selectivity of cA-PK for phosphorylation of the COOH-terminal region of L_C2 suggests that the channel activities of the two α1 subunit size forms may be differentially regulated by neurotransmitters and hormones which act through cAMP-dependent mechanisms, while both α1 subunit isoforms may be modulated by PKC, cG-PK, and calcium- and calmodulin-dependent protein kinase II.