The Crystal Structure Analysis of Group B Streptococcus Sortase C1: A Model for the “Lid” Movement upon Substrate Binding

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.

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► Access to the active site of pilus-specific sortases is blocked by the presence of a “lid” loop, whose displacement was suggested to be essential for the pilus-specific sortase function.
► With the help of an inhibitor and substrate-mimic complex crystal structures of GBS sortase SrtC1, we suggest that the active-site catalytic Cys residue is functional even in the presence of “lid”.
► We present a model for the “lid” displacement upon the substrate LPXTG sorting motif recognition and binding to the SrtC1 active site.