The Crystal Structure Analysis of Group B Streptococcus Sortase C1: A Model for the ‘lid’ Movement upon Substrate Binding

摘要

A unique feature of the class-C-type sortases, enzymes essential for Gram-positive pilus biogenesis, is the presence of a flexible ‘lid’ anchored in the active site. However, the mechanistic details of the ‘lid’ displacement, suggested to be a critical prelude for enzyme catalysis, are not yet known. This is partly due to the absence of enzyme-substrate and enzyme-inhibitor complex crystal structures. We have recently described the crystal structures of the Streptococcus agalactiae SAG2603 V/R sortase SrtC1 in two space groups (type II and type III) and that of its ‘lid’ mutant and proposed a role of the ‘lid’ as a protector of the active site hydrophobic environment. Here, we report the crystal structures of SAG2603 V/R sortase C1 in a different space group (type I) and that of its complex with a small-molecule cysteine protease inhibitor. We observe that the catalytic Cys residue is covalently linked to the small molecule inhibitor without lid displacement. However, the type I structure provides a view of the sortase SrtC1 lid displacement, while having structural elements similar to a substrate-sorting motif suitably positioned in the active site. We propose that these major conformational changes, seen in the presence of a substrate mimic in the active site, may represent universal features of class C sortase substrate recognition and enzyme activation.