C-TYPE INACTIVATION OF A VOLTAGE-GATED K+ CHANNEL OCCURS BY A COOPERATIVE MECHANISM

The lymphocyte voltage-gated K+ channel, Kv1.3, inactivates by a C-typ e process. We have elucidated the molecular basis for this process usi ng a kinetic analysis of wild-type and mutant (A413V) Kv1.3 homo- and heteromultimeric currents in a mammalian lymphoid expression system. T he medians of the measured inactivation time constants for wild-type a nd A413V homotetrameric currents are 204 and 4 ms, respectively. Co-ex pression of these subunits produces heteromultimeric channels manifest ing inactivation kinetics intermediate between those of wild-type and A413V homomultimers. We have considered several models in which each s ubunit acts either independently or cooperatively to produce the obser ved inactivation kinetics. The cooperative model gives excellent fits to the data for any heteromultimeric composition of subunits, clearly distinguishing it from the independent models. In the cooperative mode l, the difference in free energy between the open and inactivated stat es of the channel is invariant with subunit composition and equals sim ilar to 1.5 kcal/mol. Each subunit contributes equally to the activati on free energy for transitions between open and inactivated states, wi th an A413V subunit decreasing the free energy barrier for inactivatio n (and for recovery from inactivation) by similar to 0.6 kcal/mol. Our results are consistent with a physical model in which the outer mouth of the channel constricts during C-type inactivation (G. Yellen, D. S odickson, T. Chen, and M. E. Jurman, 1994, Biophys. J. 66:1068-1075).