Crystal structure of cytotoxin protein suilysin from Streptococcus suis

Cholesterol-dependent cytolysins(CDC)are pore forming toxins.A prototype of the CDC family members is perfringolysin O(PFO),which directly binds to the cell membrane enriched in cholesterol,causing cell lysis.However,an exception of this general observation is intermedilysin(ILY)of Streptococcus intermedius,which requires human CD59 as a receptor in addition to cholesterol for its hemolytic activity.A possible explanation of this functional difference is the conformational variation between the C-terminal domains of the two toxins,particularly in the highly conserved undecapeptide termed tryptophan rich motif.Here,we present the crystal structure of suilysin,a CDC toxin from the infectious swine pathogen Streptococcus suis.Like PFO,suilysin does not require a host receptor for hemolytic activity;yet the crystal structure of suilysin exhibits a similar conformation in the tryptophan rich motif to ILY.This observation suggests that the current view of the structure-function relationship between CDC proteins and membrane association is far from complete.

An open conformation determined by a structural switch for 2A protease from coxsackievirus A16

Coxsackievirus A16 belongs to the family Picornaviridae,and is a major agent of hand-foot-and-mouth disease that infects mostly children,and to date no vaccines or antivi-ral therapies are available.2A protease of enterovirus is a nonstructural protein and possesses both self-cleavage activity and the ability to cleave the eukaryotic translation initiation factor 4G.Here we present the crystal structure of coxsackievirus A162A protease,which interestingly forms hexamers in crystal as well as in solution.This structure shows an open conformation,with its active site accessible,ready for substrate binding and cleav-age activity.In conjunction with a previously reported“closed”state structure of human rhinovirus 2,we were able to develop a detailed hypothesis for the conforma-tional conversion triggered by two“switcher”residues Glu88 and Tyr89 located within the bll2-cII loop.Substrate recognition assays revealed that amino acid residues P1′,P2 and P4 are essential for substrate specificity,which was verifi ed by our substrate binding model.In addition,we compared the in vitro cleavage effi ciency of 2A pro-teases from coxsackievirus A16 and enterovirus 71 upon the same substrates by fl uorescence resonance energy transfer(FRET),and observed higher protease activity of enterovirus 71 compared to that of coxsackievirus A16.In conclusion,our study shows an open conformation of coxsackievirus A162A protease and the underlying mechanisms for conformational conversion and substrate specifi city.These new insights should facilitate the future rational design of effi cient 2A protease inhibitors.