A novel and simple method of production and biophysical characterization of a mini-membrane protein, Ost4p: a subunit of yeast oligosaccharyl transferase.

Asparagine-linked glycosylation is an essential and highly conserved protein modification reaction. In eukaryotes, oligosaccharyl transferase (OT), a multi-subunit membrane-associated enzyme complex, catalyzes this reaction in newly synthesized proteins. In Saccharomyces cerevisiae, OT consists of nine nonidentical membrane proteins. Ost4p, the smallest subunit, bridges the catalytic subunit Stt3p with Ost3p. Mutation of transmembrane residues 18-24 in Ost4p has negative effect on OT activity, disrupts the Stt3p-Ost4p-Ost3p complex, results in temperature-sensitive phenotype, and hypoglycosylation. Heterologous expression and purification of integral membrane proteins are the bottleneck in membrane protein research. The authors report the cloning, successful overexpression and purification of recombinant Ost4p with a novel but simple method producing milligram quantities of pure protein. GB1 protein was found to be the most suitable tag for the large scale production of Ost4p. The cleavage of Ost4p conveniently leaves GB1 protein in solution eliminating further purification. The precipitated pure Ost4p is reconstituted in appropriate membrane mimetic. The recombinant protein is highly helical as indicated by the far-UV CD spectrum. The well-dispersed heteronuclear single quantum coherence spectrum indicates that this minimembrane protein is well-folded. The successful production of pure recombinant Ost4p with a novel yet simple method may have important ramification for the production of other membrane proteins.