Structure of Thermotoga maritima stationary phase survival protein SurE: Anovel acid phosphatase

Background: The rpoS, nlpD, pcm, and surE genes are among many whose expres sion is induced during the stationary phase of bacterial growth. rpoS codes for the stationary-phase RNA polymerase sigma subunit, and nlpD codes for a lipoprotein. The pcm gene product repairs damaged proteins by converting the atypical isoaspartyl residues back to L-aspartyls. The physiological an d biochemical functions of surE are unknown, but its importance in stress i s supported by the duplication of the surE gene in E. coli subjected to hig h-temperature growth. The pcm and su E genes are highly conserved In bacter ia, archaea, and plants. Results: The structure of SurE from Thermotoga maritima was determined at 2 .0 Angstrom. The SurE monomer is composed of two domains; a conserved N-ter minal domain, a Rossman fold, and a C-terminal oligomerization domain, a ne w fold. Monomers form a dimer that assembles into a tetramer. Biochemical a nalysis suggests that SurE is an acid phosphatase, with an optimum pH of 5. 5-6.2. The active site was identified in the N-terminal domain through anal ysis of conserved residues. Structure-based site-directed point mutations a bolished phosphatase activity. T. maritima SurE intra- and intersubunit sal t bridges were identified that may explain the SurE thermostability. Conclusions: The structure of SurE provided information about the protein's fold, oligomeric state, and active site. The protein possessed magnesium-d ependent acid phosphatase activity, but the physiologically relevant substr ate(s) remains to be identified. The importance of three of the assigned ac tive site residues in catalysis was confirmed by site-directed mutagenesis.