Plant adenosine 5'-phosphosulphate reductase: the past, the present, and the future

The sulphate assimilation pathway provides reduced sulphur for the synthesis of the amino acids cysteine and methionine. These are the essential building blocks of proteins and further sources of reduced sulphur for the synthesis of coenzymes and various secondary compounds. Several recent reports identified the adenosine 5'-phosphosulphate reductase (APR) as the enzyme with the greatest control over the pathway. In this review, a short historical excursion into the investigations of sulphate assimilation is given with emphasis on the proposed alternative pathways to APR, via 'bound sulphite' or via PAPS reductase. The evolutionary past of APR is reviewed, based on phylogenetic analysis of APR and PAPS reductase sequences. Furthermore, recent biochemical analyses of APR that identified an iron-sulphur centre as a cofactor, proposed functions for different protein domains, and addressed the enzyme mechanism are summarized. Finally, questions that have to be addressed in order to improve understanding of the molecular mechanism and regulation of APR have been identified.