SummaryHelicases play essential roles in nearly all DNA metabolic transactions and have been implicated in a variety of human genetic disorders. A hallmark of these enzymes is the existence of a set of highly conserved amino acid sequences termed the`helicase motifs' that were hypothesized to be critical for helicase function. These motifs are shared by another group of enzymes involved in chromatin remodelling. Numerous structure±function studies, targeting highly conserved residues within the helicase motifs, have been instrumental in uncovering the functional signi®-cance of these regions. Recently, the results of these mutational studies were augmented by the solution of the three-dimensional crystal structure of three different helicases. The structural model for each helicase revealed that the conserved motifs are clustered together, forming a nucleotide-binding pocket and a portion of the nucleic acid binding site. This result is gratifying, as it is consistent with structure±func-tion studies suggesting that all the conserved motifs are involved in the nucleotide hydrolysis reaction. Here, we review helicase structure±function studies in the light of the recent crystal structure reports. The current data support a model for helicase action in which the conserved motifs de®ne an engine that powers the unwinding of duplex nucleic acids, using energy derived from nucleotide hydrolysis and conformational changes that allow the transduction of energy between the nucleotide and nucleic acid binding sites. In addition, this ATP-hydrolysing engine is apparently also associated with proteins involved in chromatin remodelling and provides the energy required to alter protein-DNA structure, rather than duplex DNA or RNA structure.

The Arabidopsis (Arabidopsis thaliana) genome encodes 51 proteins annotated as serine carboxypeptidase-like (SCPL) enzymes. Nineteen of these SCPL proteins are highly similar to one another, and represent a clade that appears to be unique to plants. Two of the most divergent proteins within this group have been characterized to date, sinapoyl-glucose (Glc):malate sinapoyltransferase and sinapoyl-Glc:choline sinapoyltransferase. The fact that two of the least related proteins within this clade are acyltransferases rather than true serine carboxypeptidases suggests that some or all of the remaining members of this group may have similar activities. The gene that encodes sinapoyl-Glc:malate sinapoyltransferase (sinapoyl-Glc accumulator1 [SNG1]: At2g22990) is one of five SCPL genes arranged in a cluster on chromosome 2. In this study, an analysis of deletion mutant lines lacking one or more genes in this SCPL gene cluster reveals that three of these genes also encode sinapoyl-Glcdependent acyltransferases. At2g23000 encodes sinapoyl-Glc:anthocyanin acyltransferase, an enzyme that is required for the synthesis of the sinapoylated anthocyanins in Arabidopsis. At2g23010 encodes an enzyme capable of synthesizing 1,2-disinapoyl-Glc from two molecules of sinapoyl-Glc, an activity shared by SNG1 and At2g22980. Sequence analysis of these SCPL proteins reveals pairwise percent identities that range from 71% to 78%, suggesting that their differing specificities for acyl acceptor substrates are due to changes in a relatively small subset of amino acids. The study of these SCPL proteins provides an opportunity to examine enzyme structure-function relationships and may shed light on the role of evolution of hydroxycinnamate ester metabolism and the SCPL gene family in Arabidopsis and other flowering plants.