Structures and Functions of Calcium Channel β Subunits
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Calcium channel β subunits have profound effects on how α1 subunits perform. In this article we summarize our present knowledge of the primary structures of β subunits as deduced from cDNAs and illustrate their different properties. Upon co-expression with α1 subunits, the effects of β subunits vary somewhat between L-type and non-L-type channels mostly because the two types of channels have different responses to voltage which are affected by β subunits, such as long-lasting prepulse facilitation of α1C (absent in α1E) and inhibition by G protein βγ dimer of α1E, absent in α1C. One β subunit, a brain β2a splice variant that is palmitoylated, has several effects not seen with any of the others, and these are due to palmitoylation. We also illustrate the finding that functional expression of α1 in oocytes requires a β subunit even if the final channel shows no evidence for its presence. We propose two structural models for Ca2+ channels to account for “α1 alone” channels seen in cells with limited β subunit expression. In one model, β dissociates from the mature α1 after proper folding and membrane insertion. Regulated channels seen upon co-expression of high levels of β would then have subunit composition α1β. In the other model, the “chaperoning” β remains associated with the mature channel and “α1 alone” channels would in fact be α1β channels. Upon co-expression of high levels of β the regulated channels would have composition [α1β]β.
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