Kinetic modification of the α1I subunit-mediated T-type Ca2+ channel by a human neuronal Ca2+ channel γ subunit

Voltage-sensitive Ca 2+ channels (VSCCs) are often heteromultimeric complexes. The VSCC subtype specifically expressed by skeletal muscle has long been known to contain a γ subunit, γ 1 , that is only expressed in this tissue. Recent work, initiated by the identification of the mutation present in the stargazer mouse, has led to the identification of a series of novel potential Ca 2+ channel γ subunits expressed in the CNS. Based on bioinformatic techniques we identified and cloned the human γ 2 , γ 3 and γ 4 subunits. TaqMan analysis was used to quantitatively characterise the mRNA expression patterns of all the γ subunits. All three subunits were extensively expressed in adult brain with overlapping but subunit-specific distributions. γ 2 and γ 3 were almost entirely restricted to the brain, but γ 4 expression was seen in a broad range of peripheral tissues. Using a myc epitope the γ 2 subunit was tagged both intracellularly at the C-terminus and on a predicted extracellular site between the first and second transmembrane domains. The cellular distribution was then examined immunocytochemically, which indicated that a substantial proportion of the cellular pool of the γ 2 subunit was present on the plasma membrane and provided initial evidence for the predicted transmembrane topology of the γ subunits. Using co-transfection techniques we investigated the functional effects of each of the γ subunits on the biophysics of the T-type VSCC encoded by the α 1I subunit. This revealed a substantially slowed rate of deactivation in the presence of γ 2 . In contrast, there was no significant corresponding effect of either γ 3 or γ 4 on α 1I subunit-mediated currents.