A novel MaxiK splice variant exhibits dominant-negative properties for surface expression

We identified a novel MaxiK alpha subunit splice variant (SV1) from rat myo metrium that is also present in brain. SV1 has a 33-amino acid insert in th e S1 transmembrane domain that does not alter S1 overall hydrophobicity, bu t makes the S0-S1 linker longer. SV1 was transfected in HEK293T cells and s tudied using immunocytochemistry and electrophysiology. In non-permeabilize d cells, N-terminal c-Myc- or C-terminal green fluorescent protein-tagged S V1 displayed no surface labeling or currents. The lack of SV1 functional ex pression was due to endoplasmic reticulum (ER) retention as determined by c olabeling experiments with a specific ER marker. To explore the functional role of SV1, we coexpressed SV1 with the alpha (human SLO) and beta1 (KCNMB 1) subunits of the MaxiK channel. Coexpression of SV1 inhibited surface exp ression of alpha and beta1 subunits similar to 80% by trapping them in the ER. This inhibition seems to be specific for MaxiK channel subunits since S V1 was unable to prevent surface expression of the Kv4.3 channel or to inte ract with green fluorescent protein. These results indicate a dominant-nega tive role of SV1 in MaxiK channel expression. Moreover, they reveal down-re gulation by splice variants as a new mechanism that may contribute to the d iverse levels of MaxiK channel expression in non-excitable and excitable ce lls.