beta subunit reshuffling modifies N- and P/Q-type Ca2+ channel subunit compositions in lethargic mouse brain

Neuronal voltage-dependent Ca2+ channels are heteromultimers of alpha(1), b eta, and alpha(2)delta subunits, and any one of five alpha(1) subunits (alp ha(1A-E)) may associate with one of four beta subunits (beta(1-4)) The spec ific alpha(1)-beta combination assembled determines single-channel properti es, while variation in the proportion of each combination contributes to th e functional diversity of neurons. The mouse mutant lethargic (lh) exhibits severe neurological defects due to a mutation that deletes the alpha(1) su bunit interaction domain of the beta(4) subunit. Since beta subunits regula te critical alpha(1) subunit properties in heterologous expression systems, loss of beta(4) in lethargic could dramatically alter channel localization and behavior unless beta(1-3), subunits can be used as substitutes in vivo . Here we demonstrate increased steady-state associations of alpha(1A) and alpha(1B) with the remaining beta(1-3), subunits, without significant chang es in beta(1-3), mRNA abundance. The immunolocalization of alpha(1A) and al pha(1B) protein in lethargic brain is indistinguishable from wild-type by l ight microscopy. Furthermore, the measurement of large-amplitude beta-type currents in dissociated lethargic Purkinje neurons indicates that these alp ha(1A)-containing channels retain regulation by beta subunits. We conclude that several properties of alpha(1A) and alpha(1B) proteins are not uniquel y regulated by beta(4) in vivo and may be rescued by beta(1-3) subunit resh uffling. The complex neurological manifestation of the lethargic mutation t herefore emerges from loss of beta(4) coupled with the widespread pairing o f surrogate beta subunits with multiple Ca2+ channel subtypes. The existenc e of beta subunit reshuffling demonstrates that molecular plasticity of Ca2 + channel assembly, a normal feature of early brain development, is retaine d in the mature brain.