CONTRIBUTION OF SIALIC-ACID TO THE VOLTAGE-DEPENDENCE OF SODIUM-CHANNEL GATING - A POSSIBLE ELECTROSTATIC MECHANISM

A potential role for sialic acid in the voltage-dependent gating of ra t skeletal muscle sodium channels (rSkM1) was investigated using Chine se hamster ovary (CHO) cells stably transfected with rSkM1. Changes in the voltage dependence of channel gating were observed after enzymati c (neuraminidase) removal of sialic acid from cells expressing rSkM1 a nd through the expression of rSkM1 in a sialylation-deficient cell lin e (lec2). The steady-state half-activation voltages (V-a) of channels under each condition of reduced sialylation were similar to 10 mV more depolarized than control channels. The voltage dependence of the time constants of channel activation and inactivation were also shifted in the same direction and by a similar magnitude. In addition, recombina nt deletion of likely glycosylation sites from the rSkM1 sequence resu lted in mutant channels that gated at voltages up to 10 mV more positi ve than wild-type channels. Thus three independent means of reducing c hannel sialylation show very similar effects on the voltage dependence of channel gating. Finally, steady-state activation voltages for chan nels subjected to reduced sialylation conditions were much less sensit ive to the effects of external calcium than those measured under contr ol conditions, indicating that sialic acid directly contributes to the negative surface potential. These results are consistent with an elec trostatic mechanism by which external, negatively charged sialic acid residues on rSkM1 alter the electric field sensed by channel gating el ements.