The role of the divergent amino and carboxyl domains on the inactivation properties of potassium channels derived from the Shaker gene of Drosophila

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Abstract
Several products generated from the Drosophila Shaker gene by alternative splicing predict a group of similar proteins with an identical central and variable amino and carboxyl domains. We have constructed 9 Sh cDNAs combining 3 different 5′ domains with 3 different 3′ domains. RNA transcribed from 6 of these cDNAs induce K+ currents in Xenopus oocytes. All currents share similar properties of voltage dependence, potassium selectivity, and block by 4-AP, TEA, and charybdotoxin. These properties presumably result from a channel core formed by the identical central region of the proteins. The currents differ in macroscopic inactivation kinetics. Five RNAs induced K+ currents which inactivate, each at distinct rates, during short depolarizations. The sixth RNA induces a current that essentially does not inactivate unless depolarized for many seconds. This raises the possibility that Sh may encode nontransient as well as transient K+ currents. Analysis of currents produced by the various combinations suggests that the divergent amino domains influence the stability of a first, nonabsorbing, inactivated state. This results in striking differences in the probability of channel reopening, as observed in single-channel recordings, of those channels with identical carboxyl but different amino domains. Furthermore, based on macroscopic analysis of the currents, we suggest that the primary role of the carboxyl domains is to influence the relative stability between the first and a second inactivated state. The second inactivated state is essentially absorbing, and recovery from this state is very slow. The observed differences in the rates of recovery from inactivation of channels containing different carboxyl domains reflect differences in the rates at which they enter this second inactivated state.