Direct interaction with a nuclear protein and regulation of gene silencing by a variant of the Ca2+-channel β4 subunit

The beta subunits of voltage-gated Ca2+ channels are known to be regulators of the channels' gating properties. Here we report a striking additional function of a beta subunit. Screening of chicken cochlear and brain cDNA libraries identified beta(4c), a short splice variant of the beta(4) subunit. Although beta(4c) occurs together with the longer isoforms beta(4a) or beta(4b) in the brain, eye, heart, and lung, the cochlea expresses exclusively beta(4c). The association Of beta(4c) with the Ca2+-channel alpha(1) subunit has slight but significant effects on the kinetics of channel activation and inactivation. Yeast two-hybrid and biochemical assays revealed that beta(4c), interacts directly with the chromo shadow domain of chromobox protein 2/heterochromatin protein 1gamma (CHCB2/HP1gamma), a nuclear protein involved in gene silencing and transcriptional regulation. Coexpression of this protein specifically recruits beta(4c) to the nuclei of mammalian cells. Furthermore, beta(4c) but not beta(4a) dramatically attenuates the gene-silencing activity of chromobox protein 2/heterochromatin protein 1gamma. The beta(4C) subunit is therefore a multifunctional protein that not only constitutes a portion of the Ca2+ channel but also regulates gene transcription.