Asn-265 of frog kainate binding protein is a functional glycosylation site: implications for the transmembrane topology of glutamate receptors.

Kainate binding proteins (KBPs) from frog and goldfish brain are glycosylated, integral membrane proteins. These KBPs are homologous (35-40%) to the C-terminal half of AMPA and kainate receptors which have been shown to form glutamate-gated ion channels. We report here that the frog KBP has three functional N-glycosylation sites. Of particular interest, Asn-265, a residue located between two putative membrane spanning regions of the frog KBP, is a functional N-glycosylation site. A mutation of Ser-267 to Gly renders this site non-functional as shown using an in vitro translation system and by transient expression in human embryonic kidney (HEK 293) cells. The mutant receptor protein (S267G), when expressed in HEK cells, binds kainate with high affinity (Kd = 16 nM). These results further support a topology with three transmembrane segments for KBPs and, by sequence homology, for glutamate-gated ion channels.