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,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)1.2alpha (alpha1C) subunit. This segment, which we term the "NT inhibitory (NTI) module," potently inhibits long-NT (cardiac) isoform of alpha1C 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 beta2b subunit reproducibly modulated function and surface expression of alpha1C, despite the presence of measurable amounts of an endogenous Ca(v)beta in Xenopus oocytes. Coexpressed beta2b increased surface expression of alpha1C approximately twofold (as demonstrated by two independent immunohistochemical methods), shifted the G-V curve by approximately 14 mV, and increased P(o,max) 2.8-3.8-fold. Neither the surface expression of the channel without Ca(v)beta nor beta2b-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,max) was completely absent in the short-NT isoform and in mutants of long-NT alpha1C lacking the NTI module. We conclude that regulation of P(o,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 alpha1C and is alpha1C isoform specific.