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Journal of Neuroscience, Vol 14, 3097-3105, Copyright © 1994 by Society for Neuroscience
The minK potassium channel exists in functional and nonfunctional forms when expressed in the plasma membrane of Xenopus oocytes
EM Blumenthal and LK Kaczmarek
Interdepartmental Neuroscience Program, Yale University School of Medicine, New Haven, Connecticut 06510.
The minK protein induces a slowly activating voltage-dependent potassium
current when expressed in Xenopus oocytes. In order to measure the levels
of minK protein in the plasma membrane, we have modified the minK gene by
inserting a 9 amino acid epitope into the N- terminal domain of the protein
sequence. When intact live oocytes are injected with the modified minK RNA
and subsequently incubated with an antibody to this epitope, specific
binding is detected, indicating that the N-terminal domain is
extracellular. We found that when oocytes are injected with amounts of minK
mRNA up to 50 ng, the levels of protein at the surface are proportional to
the amount of injected mRNA. In contrast, the amplitude of the minK current
recorded in the oocytes saturates at 1 ng of injected mRNA. Although the
amplitude of the currents is not altered by increasing mRNA levels above 1
ng, the kinetics of activation of the current differ in oocytes with high
or low levels of minK RNA. In particular, activation is slower with higher
levels of minK protein in the plasma membrane. Finally, we find that
increasing intracellular cAMP levels, which increases the amplitude of minK
currents, does not alter surface expression of the minK protein but
produces a small increase in the rate of activation of the current. Our
results support a model in which minK protein forms functional potassium
channels by association with a factor endogenous to the oocyte.
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