Biochemical study of sortase E2 from Streptomyces mobaraensis and determination of transglutaminase cross-linking sites.

Distinct streptomycetes such as Streptomyces mobaraensis produce the protein cross-linking enzyme transglutaminase. Bioinformatic analysis predicted the occurrence of seven sortases exerting transpeptidation reactions similarly to transglutaminase. Here， we report the production and characterization of sortase E2 (Sm-SrtE2) solubilized by removal of its membrane anchor domain. Sm-SrtE2 activity was measured using pentapeptides predicted to be cell wall sorting signals of putative sortase substrate proteins. Preferred linkage to Gly by Sm-SrtE2 was in the order LAETG>LAHTG>LAQTG~LANTG>LARTG. Chaplin 1 from S. mobaraensis was further demonstrated to be an excellent substrate of both the intrinsic Sm-SrtE2 and transglutaminase. The unexpected discovery showing Gln-62 and Gln-65 of Δ -Sm-SrtE2 as transglutaminase cross-linking sites suggests that low enzyme stability might be due to anchor domain truncation and a disordered N terminus.