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Structural determinants regulating surface expression and function of Kv-related ion channels Nazzari, Hamed

Abstract

To date, the mechanisms and structural determinants which contribute to the regulation of ion channel trafficking, surface expression and function have only been limitedly explored. Through biophysical and molecular characterization of the hyperpolarization-activated cyclic-nucleotide gated channel 2 (HCN2), we have identified a four-amino acid motif (EEYP) in the B-helix of the cyclic-nucleotide binding domain (CNBD) that strongly promotes channel export from the endoplasmic reticulum (ER) and cell surface expression but does not contribute to the inhibition of channel opening. We further demonstrate that this motif augments a step in the trafficking pathway and/or the efficiency of correct folding and assembly. The role of post-translational modifications, specifically N-linked glycosylation, has also been investigated in two different HCN isoforms. All four mammalian HCN channel isoforms have been shown to undergo N-linked glycosylation in the brain. HCN channels have further been suggested to require N-glycosylation for function, a provocative finding that would make them unique in the voltage-gated potassium channel superfamily. Here, we show that both the HCN1 and HCN2 isoforms are also predominantly N-glycosylated in the embryonic heart, where they are found in significant amounts and where HCN-mediated currents are known to regulate beating frequency. Surprisingly, we find that N-glycosylation is not required for HCN2 function, although its cell surface expression is highly dependent on the presence of N-glycans. Comparatively, disruption of N-glycosylation only modestly impacts cell surface expression of HCN1 and leaves permeation and gating functions almost unchanged. The evolutionary significance of this isoforms specific regulation is also examined. Finally, the role of palmitoylation in the regulation of Kv4 channels is examined. Using acylbiotin exchange (ABE) chemistry we are able demonstrate that Kv4.2 is present as a palmitoylated protein in both rat cortical neurons and COS-7 cells. Through mutational analysis of the twelve intracellular cysteine residues within Kv4.2, we were able to localize the site of palmitoylation to the intracellular COOH-terminus. Palmitoylation of Kv4.2 does not contribute to the regulation of activation and inactivation gating parameters. Rather, inhibition of palmitoylation through either mutation of COOH-terminal cysteine residues or the pharmacological agent 2-bromopalmitate results in significant reductions in overall current density measurements.

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