Squash glycerol-3-phospate-1-acyltransferase has been crystallized and the structure of the enzyme determined, at 1.9-Å resolution, using multiple isomorphous replacement of the wild type and a series of individual cysteine mutants. Competitive in vitro substrate selectivity assays have been established that differentiate between selective and non-selective forms of the enzyme. Particular care was taken to use near-physiological concentrations of both substrates. Clear substrate selectivity can be demonstrated with the natural substrate acyl-acyl carrier protein but not with the substrate analogue acyl-CoA. The use of site-directed mutagenesis, coupled to three-dimensional structural determinations, should provide a rational basis for elucidating structural components important in determining the substrate selectivity of this enzyme.

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