Ancillary subunits of the Kv4 potassium channels.
We have recently cloned a group of calcium binding proteins that have been called KChIPs (K+ channel interaction proteins), which have been shown to be subunits of Kv4 potassium channels. We have identified a form which co-localizes with Kv4 channels in the heart, and shown that this his isoform interacts with the pore of the Kv4 channel. Future work will explore the mechanisms by which these proteins interact with, and modulation Kv4 channel function
Cardiac Ito has at least two distinct phenotypes within the mammalian heart, a form found in the left ventricular epicardium and the right ventricle that shows fast recovery from inactivation, and a from predominant in the left ventricular endocardium the recovers more slowly. Using a combination of immunolocalization (shown as a cut-away of the ferret ventricles) and electrophysiology, we showed that the fast recovering Ito (in the red boxes) co-localizes in the heart with the closely related Kv4.2 and Kv4.3 potassium channels (predominantly green & red), and the slow recovering Ito localizes with the Kv1.4 channel (blue boxes, blue stain in the heart).
Immunoblot (left) of heart protein extracts tested with an antibody specific to KChIP2b (alignment of the isoforms at right) shows that this isoform is present in the right ventricle (RV), and the LV epi, but not in the inner portion of the LV (endo). This is the same localization pattern found for Kv4.3 and Kv4.2, suggesting that KChIP2b may be a part of this channel complex. HEK-2, 2a, and 2b are cell lines that have been transfected with each of these cDNAs, showing that the KChIP2b antibody is isoform-specific.
KChIP2b reconstitutes fast recovery from inactivation. Co-expression of Kv4.3 and KChIP2b in a Xenopus oocyte shows that KChIP2b speeds recovery from inactivation. A pore mutant of Kv4.3 shows a different phenotype when expressed with KChIP2b, suggesting pore structure is an important factor in determining Kv4 recovery properties.
Schematic of a voltage-gated potassium channel alpha subunit monomer, showing the position of the pore mutations.
The immunofluorescence data was generated primarily by Dr. Brahmajothi Mulugu, and the myocyte data by Dr. Donald Campbell, and appeared in an article by Brahmajothi, et al. The ferret KChIP isoforms and antibodies were isolated as part of the thesis project of Sangita Patel, and have recently been published in the Journal of Physiology. The Kv4.3/KChIP2b expression data was generated by my colleague, Dr. Shimin Wang and can be found in BBRC.
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