New short splice variants of the human cardiac Cavbeta2 subunit: redefining the major functional motifs implemented in modulation of the Cav1.2 channel.

Two new short splice variants of the Ca2+ channel beta2 subunit were cloned from human heart poly(A)(+) mRNA. The 410-amino acid beta2f subunit is encoded by exons 1A, 2A, 3, 4, 12, 13, and 14 of the human Cavbeta2 gene and lacks the protein kinase A phosphorylation site, the beta-interaction domain (De Waard, M., Pragnell, M., and Campbell, K. P. (1994) Neuron 13, 495-503), 40% of the beta-SH3 domain, and 73% of the guanylate kinase domain of the putative membrane-associated guanylate kinases module (McGee, A. W., Nunziato, D. A., Maltez, J. M., Prehoda, K. E., Pitt, G. S., and Bredt, D. S. (2004) Neuron 42, 89-99), and helix alpha3 of the alpha1-subunit binding pocket (Van Petegem F., Clark, K. A., Chatelain, F. C., and Minor, D. L., Jr. (2004) Nature 429, 671-675). The beta2g transcript has two potential initiation codons. With the second ATG codon, it generates the 164-amino acid beta2Deltag subunit encoded essentially by the distal part of exon 14, and thus beta2Deltag completely lacks any of the above motifs. Immunoprecipitation analysis confirmed stable association of beta2f and beta2Deltag with the alpha1C subunit. The plasma membrane localization of beta2f and beta2Deltag was substantially increased by co-expression of the alpha1C,77 and alpha2delta subunits. In COS1 cells, beta2f and beta2Deltag increased plasma membrane targeting of the pore-forming alpha1C subunit and differentially facilitated (beta2f > beta2Deltag) the voltage gating of otherwise silent Cav1.2 channels. We conclude that it is unlikely that the beta-interaction domain, membrane-associated guanylate kinases module, and the alpha1-subunit binding pocket helix alpha3 are essential for the interaction of the alpha1C and beta2 subunits and suggest that in addition to the alpha1-subunit binding pocket helices alpha5 and alpha8, a yet unresolved C-terminal beta2 region plays a crucial role.