Glycosylation of GIRK1 at Asn(119) and ROMK1 at Asn(117) has different consequences in potassium channel function

GIRK (G protein-gated inward rectifier K+ channel) proteins play critical f unctional roles in heart and brain physiology, Using antibodies directed to either GIRK1 or GIRK4, site-directed mutagenesis, and specific glycosidase s, we have investigated the effects of glycosylation in the biosynthesis an d heteromerization of these proteins expressed in oocytes, Both GIRK1 and G IRK4 have one extracellular consensus N-glycosylation site. Using chimeras between GIRK1 and GIRK4 as well as a GIRK1 N-glycosylation mutant, we repor t that GIRK1 was glycosylated at Asn(119), whereas GIRK4 was not glycosylat ed at Asn(132). GIRK1 membrane-spanning domain 1 was required for optimal g lycosylation at Asn(119) because a chimera that contained GIRK4 membrane-sp anning domain I significantly reduced the addition of a carbohydrate struct ure at this site. This finding may partly account for the reason that GIRK4 is not glycosylated at Asn132, either as a homomer or when coexpressed wit h GIRK1, When the GIRK1(N119Q) mutant was coexpressed with GIRK4, the bioph ysical properties of the heteromeric channel and the magnitude of the agoni st-induced currents were similar to those of controls, Thus, N-glycosylatio n of GIRK1 at Asn(119) does not appear to affect its physical association w ith GIRK4, the routing of the heteromer to the cell surface, or heteromeric channel function, unlike the dramatic functional effects of N-glycosylatio n of ROMK1 at Asn117 (Schwalbe, R. A. Wang, Z., Wible, B. A. and Brown, A. M. (1995) J. Biol. Chem. 270, 15336-15340).