Regulation of Maximal Open Probability Is a Separable Function of Ca(v)β Subunit in L-type Ca(2+) Channel, Dependent on NH(2) Terminus of α(1C) (Ca(v)1.2α)

β subunits (Ca(v)β) increase macroscopic currents of voltage-dependent Ca(2+) 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)β-induced increase in macroscopic current crucially depends on the initial segment of the cytosolic NH(2) terminus (NT) of the Ca(v)1.2α (α(1C)) subunit. This segment, which we term the “NT inhibitory (NTI) module,” potently inhibits long-NT (cardiac) isoform of α(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) α(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 β(2b) subunit reproducibly modulated function and surface expression of α(1C), despite the presence of measurable amounts of an endogenous Ca(v)β in Xenopus oocytes. Coexpressed β(2b) increased surface expression of α(1C) approximately twofold (as demonstrated by two independent immunohistochemical methods), shifted the G-V curve by ∼14 mV, and increased P(o,max) 2.8–3.8-fold. Neither the surface expression of the channel without Ca(v)β nor β(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,max) was completely absent in the short-NT isoform and in mutants of long-NT α(1C) lacking the NTI module. We conclude that regulation of P(o,max) is a discrete, separable function of Ca(v)β. In Ca(v)1.2, this action of Ca(v)β depends on NT of α(1C) and is α(1C) isoform specific.