Unique modulation of L-type Ca2+ channels by short auxiliary beta1d subunit present in cardiac muscle.

Recent studies have identified a growing diversity of splice variants of auxiliary Ca2+ channel Ca(v)beta subunits. The Ca(v)beta(1d) isoform encodes a putative protein composed of the amino-terminal half of the full-length Ca(v)beta(1) isoform and thus lacks the known high-affinity binding site that recognizes the Ca2+ channel alpha1-subunit, the alpha-binding pocket. The present study investigated whether the Ca(v)beta(1d) subunit is expressed at the protein level in heart, and whether it exhibits any of the functional properties typical of full-length Ca(v)beta subunits. On Western blots, an antibody directed against the unique carboxyl terminus of Ca(v)beta(1d) identified a protein of the predicted molecular mass of 23 kDa from canine and human hearts. Immunocytochemistry and surface-membrane biotinylation experiments in transfected HEK-293 cells revealed that the full-length Ca(v)beta(1b) subunit promoted membrane trafficking of the pore-forming alpha1C (Ca(v)1.2)-subunit to the surface membrane, whereas the Ca(v)beta(1d) subunit did not. Whole cell patch-clamp analysis of transfected HEK-293 cells demonstrated no effect of coexpression of the Ca(v)beta(1d) with the alpha1C-subunit compared with the 15-fold larger currents and leftward shift in voltage-dependent activation induced by full-length Ca(v)beta(1b) coexpression. In contrast, cell-attached patch single-channel studies demonstrated that coexpression of either Ca(v)beta(1b) or Ca(v)beta(1d) significantly increased mean open probability four- to fivefold relative to the alpha1C-channels alone, but only Ca(v)beta(1b) coexpression increased the number of channels observed per patch. In conclusion, the Ca(v)beta(1d) isoform is expressed in heart and can modulate the gating of L-type Ca2+ channels, but it does not promote membrane trafficking of the channel complex.