A specific N-terminal residue in Kv1.5 is required for upregulation of the channel by SAP97

We have previously reported that SAP97 enhancement of hKv1.5 currents requires an intact Kv1.5 N-terminus and is independent of the PDZ-binding motif at the C-terminus of the channel [J. Eldstrom, W.S. Choi, D.F. Steele, D. Fedida, SAP97 increases Kv1.5 currents through an indirect N-terminal mechanism, FEBS Lett. 547 (2003) 205-211]. Here, we report that an interaction between the two proteins can be detected under certain conditions but their interaction is irrelevant to the enhancement of channel expression. Instead, a threonine residue at position 15 in the hKv1.5 N-terminus is critically important. Mutation of this residue, which lies within a consensus site for phosphorylation by protein kinase C, to an alanine, completely abrogated the effect of SAP97 on channel expression. Although we were unable to detect phosphorylation of this residue, specific inhibition of kinase C by Calphostin C eliminated the increase in wild-type hKv1.5 currents associated with SAP97 overexpression suggesting a role for this kinase in the response.