Activation of the calcium-permeable cation channel CD20 by α subunits of the G(i) protein

When the calcium-permeable cation channel CD20 is expressed in Balb/c 3T3 cells, it is activated by insulin-like growth factor-I (IGF-I) via the IGF-I receptor (Kanzaki, M., Nie, L., Shibata, H., and Kojima, I. (1997) J. Biol. Chem. 272, 4964-4969). The present study was conducted to investigate the role of G proteins in the regulation of the CD20 channel. In the excised patch clamp mode, activation of the CD20 channel by IGF-I required GTP, Mg²⁺, and ATP in the bath solution, and removal of either GTP or ATP attenuated the activation. Non-hydrolyzable ATP could substitute for ATP, and guanyl-5'-yl thiophosphate blocked the activation of the channel by IGF-I. The CD20 channel was also activated by guanosine 5'-3-O-(thio)triphosphate, and ATP was not required for the activation. Addition of a preparation of G(i)/G(o) holoprotein purified from bovine brain activated the CD20, and the β-adrenergic receptor kinase peptide did not affect the number of channel openings induced by the G protein. The CD20 channel was stimulated by the GTP-bound form of recombinant G(i2) α subunit purified from Sf9 cells. The G(i3) α subunit was less effective, and the G(i1) α subunit had no effect. Purified recombinant β₁γ₂) subunits did not affect the activity of the channel. Finally, IGF-I-induced activation of CD20 was inhibited by an antibody against G(i2) α subunit. These findings indicate that the CD20 channel expressed in Balb/c 3T3 cells is activated by the IGF-I receptor via the α subunits of heterotrimeric G proteins.