In recent years, the development of chemical biology has been increasing focusing on efficient and mild methods for chemo-selective ligations and site-specific proteins labeling and modifications. These methods have ability to obtain large amount of post translational modifications and artificial proteins, which could not be acquired by using traditional gene cloning and recombinant protein expression strategy. In later 1990s, a new transpeptidase Sortase A was isolated from Staphylococcus Aureus, which can be used to modify proteins bearing a short recognition sequence (most usually as LPXTG or LPAAG). The active-site Cys residue of Sortase A cleaves between LPXT and G residue to produce a thioester intermediate, which can reacts with a nucleophile containing one to five Gly to afford the ligation product. Base on above-mentioned reason, the Sortase-mediated ligation has been successfully applied to many fields such as C-terminals protein modification, labeling and protein semisynthesis with high efficiency recently. Compared to traditional chemical synthesis,Sortase catalyzed semi-chemical synthesis method can preferably address the size problem of protein chemical synthesis. This mini review reports and discusses the recent important development of protein ligations, labeling and modifications by using Sortase mediated ligation method.

Contents
1 Introduction
2 The structure of Sortase and the mechanism of the transpeptidation reaction catalyzed by Sortase
3 Sortases as tools for protein ligations and modifications
3.1 Validation of the in vitro Sortase activity
3.2 Seek for the mutants of Sortase
3.3 Immobilization of Sortase
4 Application of Sortase to protein ligations and modifications
4.1 C terminal modifications
4.2 N terminal modifications
4.3 C and N terminal modifications
4.4 Preparation of unnatural N-N/C-C chimeric proteins by using Sortase
4.5 Protein immobilization
4.6 Preparation of cyclopeptides
4.7 Sortase-tag expressed protein ligation reaction
4.8 Other applications
5 Conclusion and outlook