N-linked glycosylation is the most common post-translational modification of G-protein-coupled receptors (GPCRs) and is correlated to the localization and function of the receptors depending on each receptor. However, heterogeneity of glycosylation can interfere with protein crystallization. The removal of N-linked glycosylation from membrane proteins improves the ability to crystallize these proteins. We screened 25 non-glycosylated GPCRs for functional receptor production in the methylotrophic yeast Pichia pastoris using specific ligand-receptor binding assays. We found that five clones were expressed at greater than 10 pmol/mg, 9 clones at 1-10 pmol/mg and 11 clones at less than 1 pmol/mg of membrane protein. Further optimization of culture parameters including Culture scale, induction time, pH and temperature enabled us to achieve expression of a functional human muscarinic acetylcholine receptor subtype 2 (CHRM2) with a B-max value of 51.2 pmol/mg of membrane protein. Approximately 1.9 mg of the human CHRM2 was produced from a 1-L culture. (c) 2009 Elsevier Inc. All rights reserved.