Crystal forms and subunit stoichiometry of serine hydroxymethyltransferase

Serine hydroxymethyltransferase catalyzes the formation of one-carbon units essential for anabolic processes leading to the formation of such essential cellular components as purines, pyrimidines, amino acids, and lipids. Crystal structure determinations of several forms of the enzyme are under way, and to expand the comparative scope of these studies, we have crystallized the rabbit cytosolic and mitochondrial enzymes. Crystallization of serine hydroxymethyltransferase from different sources has often been problematic, and we report studies addressing these difficulties that may have more general application. The crystal lattice symmetry and the stoichiometry of the crystal asymmetric units and of the enzyme in solution suggest that serine hydroxymethyltransferase may exist as dimers, trimers, and possibly higher order complexes and that their aggregation state is affected by ionic strength.