Complex N-glycosylation stabilizes surface expression of transient receptor potential melastatin 4b protein.

N-glycosylation is important for the function and regulation of ion channels. We examined the role of N-glycosylation of transient receptor potential melastatin (Trpm) 4b， a membrane glycoprotein that regulates calcium influx. Trpm4b was expressed in vivo in all rat tissues examined. In each tissue， Trpm4b had a different molecular mass， between ∼129 and ∼141 kDa， but all reverted to ∼120 kDa following treatment with peptide:N-glycosidase F， consistent with N-glycosylation being the principal form of post-translational modification of Trpm4b in vivo. In six stable isogenic cell lines that express different levels of Trpm4b， two forms were found， high mannose， core-glycosylated and complex， highly glycosylated (HG)， with HG-Trpm4b comprising 85% of the total Trpm4b expressed. For both forms， surface expression was directly proportional to the total Trpm4b expressed. Complex N-glycosylation doubled the percentage of Trpm4b at the surface， compared with high mannose N-glycosylation. Mutation of the single N-glycosylation consensus sequence at Asn-988 (Trpm4b-N988Q)， located near the pore-forming loop between transmembrane helices 5 and 6， prevented glycosylation， but did not prevent surface expression， impair formation of functional membrane channels， or alter channel conductance. In transfection experiments， the time courses for appearance of HG-Trpm4b and Trpm4b-N988Q on the surface were similar. In experiments with cycloheximide inhibition of protein synthesis， the time course for disappearance of HG-Trpm4b from the surface was much slower than that for Trpm4b-N988Q. We conclude that N-glycosylation is not required for surface expression or channel function， but that complex N-glycosylation plays a crucial role in stabilizing surface expression of Trpm4b.