The cysteine synthase complex from plants

Abstract

Serine acetyltransferase (SAT) catalyzes the rate‐limiting step of cysteine biosynthesis in bacteria and plants and functions in association with O‐acety lserine (thiol) lyase (OAS‐TL) in the cysteine synthase complex. Very little is known about the structure and catalysis of SATs except that they share a characteristic C‐terminal hexapeptide‐repeat domain with a number of enzymatically unrelated acyltransferases. Computational modeling of this domain was performed for the mitochondrial SAT isoform from Arabidopsis thaliana , based on crystal structures of bacterial acyltransferases. The results indicate a left‐handed parallel β‐helix consisting of β‐sheets alternating with turns, resulting in a prism‐like structure. This model was challenged by site‐directed mutagenesis and tested for a suspected dual function of this domain in catalysis and hetero‐oligomerization. The bifunctionality of the SAT C‐terminus in transferase activity and interaction with OAS‐TL is demonstrated and discussed with respect to the putative role of the cysteine synthase complex in regulation of cysteine biosynthesis.