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).