The alpha 1B Ca2+ channel amino terminus contributes determinants for betasubunit-mediated voltage-dependent inactivation properties

1. Co-expression of auxiliary beta subunits with the alpha 1 beta Ca2+ chan nel subunit in COS-7 cells resulted in an increase in current density and a hyperpolarising shift in the mid-point of activation. Amongst the beta sub units, beta 2a in particular, but also beta 4 and beta 1b caused a signific ant retardation of the voltage-dependent inactivation compared to currents with alpha 1B alone, whilst no significant changes in inactivation properti es were seen for the beta 3 subunit in this system. 2. Prevention of beta 2a palmitoylation, by introducing cysteine to serine mutations (beta 2a(C3,4S)), greatly reduced the ability of beta 2a to retar d voltage-dependent inactivation. 3. Deletion of the proximal half of the alpha 1B cytoplasmic amino terminus (alpha 1B(Delta 1-55)) differentially affected beta subunit-mediated volta ge-dependent inactivation properties. These effects were prominent with the beta 2a subunit and, to a lesser extent, with beta 1b. For beta 2a, the ma jor effects of this deletion were a partial reversal of beta 2a-mediated re tardation of inactivation and the introduction of a fast component of inact ivation, not seen with full-length alpha 1B. Deletion of the amino terminus had no other major effects on the measured biophysical properties of alpha 1B when co-expressed with beta subunits. 4. Transfer of the whole alpha 1B amino terminus into alpha 1C (alpha 1bCCC C) conferred a similar retardation of inactivation on alpha 1C when co-expr essed with beta 2a to that seen in parental alpha 1B. 5. Individual (alpha 1B(Q47A) and alpha 1B(R52A)) and double (alpha 1B(R52, 54A)) point mutations within the amino terminus of alpha 1B also opposed th e beta 2a-mediated retardation of alpha 1B inactivation kinetics. 6. These results indicate that the alpha 1B amino terminus contains determi nants for beta subunit-mediated voltage-dependent inactivation properties. Furthermore, effects were beta subunit selective. As deletion of the alpha 1B amino terminus only partially opposed beta subunit-mediated changes in i nactivation properties, the amino terminus is likely to contribute to a com plex site necessary for complete beta subunit function.