Alternative splicing of a short cassette exon in alpha(1B) generates functionally distinct N-type calcium channels in central and peripheral neurons

The N-type Ca channel alpha(1B) subunit is localized to synapses throughout the nervous system and couples excitation to release of neurotransmitters. In a previous study, two functionally distinct variants of the alpha(1B) s ubunit were identified, rn alpha(1B-b) and rn alpha(1B-d), that differ at t wo loci; four amino acids [SerPheMetGly (SFMG)] in IIIS3-S4 and two amino a cids [GluThr (ET)] in IVS3-S4, These variants are reciprocally expressed in rat brain and sympathetic ganglia (Lin et al., 1997a). We now show that th e slower activation kinetics of rn alpha(1B-b) (Delta SFMG/+ET) compared wi th rn alpha(1B-d) (+SFMG/Delta ET) channels are fully accounted for by the insertion of ET in IVS3-S4 and not by the lack of SFMG in IIIS3-S4. We also show that the inactivation kinetics of these two variants are indistinguis hable. Through genomic analysis we identify a six-base cassette exon that e ncodes the ET site and with ribonuclease protection assays demonstrate that the expression of this mini-exon is essentially restricted to alpha(1B) RN As of peripheral neurons, We also show evidence for regulated alternative s plicing of a six-base exon encoding NP in the IVS3-S4 linker of the closely related alpha(1A) gene and establish that residues NP can functionally sub stitute for ET in domain IVS3-S4 of alpha(1B). The selective expression of functionally distinct Ca channel splice variants of alpha(1B) and alpha(1A) subunits in different regions of the nervous system adds a new dimension o f diversity to voltage-dependent Ca signaling in neurons that may be import ant for optimizing action potential-dependent transmitter release at differ ent synapses.