Regulation of maximal open probability is a separable function of Cavβ subunit in L-type Ca2+ channel, dependent on NH2 terminus of α1C (Cav1.2α)

beta subunits (Ca-v beta) increase macroscopic currents of voltage-dependent Ca2+ channels (VDCC) by increasing surface expression and modulating their gating, causing a leftward shift in conductance-voltage (G-V) curve and increasing the maximal open probability, P-o,P-max. In L-type Ca(v)1.2 channels, the Ca-v beta-induced increase in macroscopic current crucially depends on the initial segment of the cytosolic NH2 terminus (NT) of the Ca(v)1.2 alpha ( alpha(1C)) subunit. This segment, which we term the "NT inhibitory (NTI) module," potently inhibits long-NT (cardiac) isoform of alpha(1C) that features an initial segment of 46 amino acid residues (aa); removal of NTI module greatly increases macroscopic currents. It is not known whether an NTI module exists in the short-NT (smooth muscle/brain type) alpha(1C) isoform with a 16-aa initial segment. We addressed this question, and the molecular mechanism of NTI module action, by expressing subunits of Ca(v)1.2 in Xenopus oocytes. NT deletions and chimeras identified aa 1-20 of the long-NT as necessary and sufficient to perform NTI module functions. Coexpression of beta(2b) subunit reproducibly modulated function and surface expression of alpha(1C), despite the presence of measurable amounts of an endogenous Ca-v beta in Xenopus oocytes. Coexpressed beta(2b) increased surface expression of alpha(1C) approximately twofold (as demonstrated by two independent immunohistochemical methods), shifted the G-V curve by similar to 14 mV, and increased P-o,P- max 2.8-3.8-fold. Neither the surface expression of the channel without Ca-v beta nor beta(2b)-induced increase in surface expression or the shift in G-V curve depended on the presence of the NTI module. In contrast, the increase in P-o,P- max was completely absent in the short-NT isoform and in mutants of long-NT alpha(1C) lacking the NTI module. We conclude that regulation of P-o,P- max is a discrete, separable function of Ca-v beta. In Ca(v)1.2, this action of Ca-v beta depends on NT of alpha(1C) and is alpha(1C) isoform specific.