New short splice variants of the human cardiac Ca_vβ ₂ subunit: Redefining the major functional motifs implemented in modulation of the Ca_v1.2 channel

Two new short splice variants of the Ca²⁺ channel β₂ subunit were cloned from human heart poly(A)(+) mRNA. The 410-amino acid β_2f subunit is encoded by exons 1A, 2A, 3, 4, 12, 13, and 14 of the human Ca_vβ₂ gene and lacks the protein kinase A phosphorylation site, the β-interaction domain (De Waard, M., Pragnell, M., and Campbell, K. P. (1994) Neuron 13, 495-503), 40% of the β-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 α3 of the α₁-subunit binding pocket (Van Petegem F., Clark, K. A., Chatelain, F. C., and Minor, D. L., Jr. (2004) Nature 429, 071-075). The β_2g transcript has two potential initiation codons. With the second ATG codon, it generates the 164-amino acid β_2Δg subunit encoded essentially by the distal part of exon 14, and thus β_2Δg completely lacks any of the above motifs. Immunoprecipitation analysis confirmed stable association of β_2f and β_2Δg with the α_1C subunit. The plasma membrane localization of β_2f and β_2Δg was substantially increased by co-expression of the α_1C,77 and α₂δ subunits. In COS1 cells, β_2f and β_2Δg increased plasma membrane targeting of the pore-forming α_1C subunit and differentially facilitated (β_2f > β_2Δg) the voltage gating of otherwise silent Ca_v1.2 channels. We conclude that it is unlikely that the β-interaction domain, membrane-associated guanylate kinases module, and the α₁-subunit binding pocket helix α3 are essential for the interaction of the α_1C and β₂ subunits and suggest that in addition to the α₁-subunit binding pocket helices α5 and α8, a yet unresolved C-terminal β₂ region plays a crucial role.