β 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), beta, and alpha(2)delta subunits, and any one of five alpha(1) subunits (alpha(1A-E)) may associate with one of four beta subunits (beta(1-4)) The specific alpha(1)-beta combination assembled determines single-channel properties, while variation in the proportion of each combination contributes to the functional diversity of neurons. The mouse mutant lethargic (lh) exhibits severe neurological defects due to a mutation that deletes the alpha(1) subunit interaction domain of the beta(4) subunit. Since beta subunits regulate 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 changes in beta(1-3), mRNA abundance. The immunolocalization of alpha(1A) and alpha(1B) protein in lethargic brain is indistinguishable from wild-type by light microscopy. Furthermore, the measurement of large-amplitude beta-type currents in dissociated lethargic Purkinje neurons indicates that these alpha(1A)-containing channels retain regulation by beta subunits. We conclude that several properties of alpha(1A) and alpha(1B) proteins are not uniquely regulated by beta(4) in vivo and may be rescued by beta(1-3) subunit reshuffling. The complex neurological manifestation of the lethargic mutation therefore emerges from loss of beta(4) coupled with the widespread pairing of surrogate beta subunits with multiple Ca2+ channel subtypes. The existence of beta subunit reshuffling demonstrates that molecular plasticity of Ca2+ channel assembly, a normal feature of early brain development, is retained in the mature brain.