Glycosylation regulates the function and membrane localization of KCC4

Glycosylation plays a role in regulating many biological activities, including protein folding and cell surface expression of biomolecules. However, the importance of glycosylation for KCC4 function has not previously been demonstrated. Site-directed mutagenesis was performed on the four putative extracellular N-linked glycosylation sites of KCC4 to determine the role of these sites in KCC4 half-life, cell surface expression, and transporter activity, as well as in KCC4-dependent tumor formation. We showed that triple (N312/331/344/Q) and quadruple (N312/331/344/360/Q) mutations of N-linked glycosylation sites disrupt the N-linked glycosylation of KCC4, resulting in the accumulation of KCC4, predominantly in the endoplasmic reticulum (ER) and not at the cell surface. Further investigation indicated that mutations of the central two (N331/344/Q) N-linked glycosylation sites inhibit the membrane trafficking of KCC4. Our data suggest that the glycan moieties at the N331 and N344 sites were Endo H-resistant, complex-form structures, and that the N312 and N360 sites were Endo H-sensitive, high mannose-containing structures. Under hypotonic stress conditions, the ability to adapt to changes in intracellular chloride ion concentrations and RVD (regulatory volume decrease) activities were less efficient in cells containing the deglycosylated form of KCC4 that were not expressed at the cell surface. Deglycosylated forms of KCC4 also demonstrated decreased tumor formation and lung colonization in mouse xenografts. The difference in glycan complexity may account for the differential impact of each branch on the biological effects of KCC4. We propose that glycosylation is essential for the surface expression, stabilization, and bioactivity of KCC4.