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Heteromeric Kv1 potassium channel expression: amino acid determinants involved in processing and trafficking to the cell surface.

Jing Zhu, Itaru Watanabe, Barbara Gomez, William B Thornhill

J. Biol. Chem., 2003 Jul 11 , 278, 25558-67

Kv1.4 and Kv1.1 potassium channels are expressed in brain as mature glycoproteins that are trans-Golgi glycosylated. When expressed in cell lines these homomers had very different trans-Golgi glycosylation efficiencies and cell surface expression levels with Kv1.4 > Kv1.1 for both parameters (Zhu, J., Watanabe, I., Gomez, B., and Thornhill, W. B. (2001) J. Biol. Chem. 276, 39419-39427). This previous study identified determinants in the outer pore region of Kv1.4 and Kv1.1 that positively and negatively, respectively, affected these events when expressed as homomers. Here we investigated which subunit exhibited positive or negative effects on these processes when expressed as heteromers. Kv1.4/Kv1.1 heteromers, by coexpression or expression as tandem-linked heteromers, were expressed on the cell surface at approximately 20-fold lower levels versus Kv1.4 homomers but they were trans-Golgi glycosylated. The lower Kv1.4/Kv1.1 expression level was not rescued by Kvbeta 2.1 subunits. Thus Kv1.1 inhibited high cell surface expression and partially retained the heteromer in the endoplasmic reticulum, whereas Kv1.4 stimulated trans-Golgi glycosylation. The subunit determinants and cellular events responsible for these differences were investigated. In a Kv1.4/Kv1.1 heteromer, the Kv1.1 pore was a major negative determinant, and it inhibited high cell surface expression because it induced high partial endoplasmic reticulum retention and it decreased protein stability. Other Kv1.1 regions also inhibited high surface expression of heteromers. The Kv1.1 C terminus induced partial Golgi retention and contributed to a decreased protein stability, whereas the Kv1.1 N terminus contributed to only a decreased protein stability. Thus a neuron may regulate its cell surface K+ channel protein levels by different Kv1 subfamily homomeric and heteromeric combinations that affect intracellular retention characteristics and protein stability.