A Xenopus oocyte beta subunit: Evidence for a role in the assembly/expression of voltage-gated calcium channels that is separate from its role as a regulatory subunit

Two closely related beta subunit mRNAs (xo28 and xo32) were identified in Xenopus oocytes by molecular cloning. One or both appear to be expressed as active proteins, because: (i) injection of Xenopus beta antisense oligonucleotides, but not of sense or unrelated oligonucleotides, significantly reduced endogenous oocyte voltage-gated Ca2+ channel (VGCC) currents and obliterated VGCC currents that arise after injection of mammalian alpha(1) cRNAs (alpha(1C) and alpha(1E)); (ii) coinjection of a Xenopus beta antisense oligonucleotide and excess rat beta cRNA rescued expression of alpha(1) Ca2+ channel currents; and (iii) coinjection of mammalian alpha(1) CRNA with cRNA encoding either of the two Xenopus beta subunits facilitated both activation and inactivation of Ca2+ channel currents by voltage, as happens with most mammalian beta subunits, The Xenopus beta subunit cDNAs (beta 3xo cDNAs) predict proteins of 484 aa that differ in only 22 aa and resemble most closely the sequence of the mammalian type 3 beta subunit, We propose that ''alpha(1) alone'' channels are in fact tightly associated alpha(1) beta(3)xo channels, and that effects of exogenous beta subunits are due to formation of higher-order [alpha(1) beta]beta(n) complexes with an unknown contribution of beta 3xo. It is thus possible that functional mammalian VGCCs, rather than having subunit composition alpha(1) beta, are [alpha(1) beta]beta(n) complexes that associate with alpha(2) delta and, as appropriate, other tissue-specific accessory proteins. In support of this hypothesis, we discovered that the last 277-aa of alpha(1E) have a beta subunit binding domain, This beta binding domain is distinct from the previously known interaction domain located between repeats I and II of calcium channel alpha(1) subunits.