Structural insights into the putative bacterial acetylcholinesterase ChoE and its substrate inhibition mechanism.

Mammalian acetylcholinesterase (AChE) is well-studied, being important in both cholinergic brain synapses and the peripheral nervous systems and also a key drug target for many diseases. In contrast, little is known about the structures and molecular mechanism of prokaryotic acetylcholinesterases. We report here the structural and biochemical characterization of ChoE, a putative bacterial acetylcholinesterase from Pseudomonas aeruginosa Analysis of WT and mutant strains indicated that ChoE is indispensable for P. aeruginosa growth with acetylcholine as the sole carbon and nitrogen source. The crystal structure of ChoE at 1.35 Å resolution revealed that this enzyme adopts a typical fold of the SGNH hydrolase family. Although ChoE and eukaryotic AChEs catalyze the same reaction, their overall structures bear no similarities constituting an interesting example of convergent evolution. Among Ser-38, Asp-285, and His-288 of the catalytic triad residues, only Asp-285 was not essential for ChoE activity. Combined with kinetic analyses of WT and mutant proteins, multiple crystal structures of ChoE complexed with substrates, products, or reaction intermediate revealed the structural determinants for substrate recognition, snapshots of the various catalytic steps, and the molecular basis of substrate inhibition at high substrate concentrations. Our results indicate that substrate inhibition in ChoE is due to acetate release being blocked by the binding of a substrate molecule in a nonproductive mode. Because of the distinct overall folds and significant differences of the active site between ChoE and eukaryotic AChEs, these structures will serve as a prototype for other prokaryotic acetylcholinesterases.

The gene encoding IIAB(Man)L in Streptococcus salivarius is part of a tetracistronic operon encoding a phosphoenolpyruvate: mannose/glucose phosphotransferase system.

Glucose and mannose are transported in streptococci by the mannose-PTS (phosphoenolpyruvate:mannose phosphotransferase system), which consists of a cytoplasmic IIAB protein, called IIAB(Man), and an uncharacterized membrane permease. This paper reports the characterization of the man operon encoding the specific components of the mannose-PTS of Streptococcus salivarius. The man operon was composed of four genes, manL, manM, manN and manO. These genes were transcribed from a canonical promoter (Pman) into a 3.6 kb polycistronic mRNA that contained a 5'-UTR (untranslated region). The predicted manL gene product encoded a 35.5 kDa protein and contained the amino acid sequences of the IIA and IIB phosphorylation sites already determined from purified S. salivarius IIAB(Man)L. Expression of manL in Escherichia coli generated a 35 kDa protein that reacted with anti-IIAB(Man)L antibodies. The predicted ManM protein had an estimated size of 27.2 kDa. ManM had similarity with IIC domains of the mannose-EII family, but did not possess the signature proposed for mannose-IIC proteins from Gram-negative bacteria. From multiple alignment analyses of sequences available in current databases, the following modified IIC(Man) signature is proposed: GX3G[DNH]X3G[LIVM]2XG2[STL][LT][EQ]. The deduced product of manN was a hydrophobic protein with a predicted molecular mass of 33.4 kDa. The ManN protein contained an amino acid sequence similar to the signature sequence of the IID domains of the mannose-EII family. manO encoded a 13.7 kDa protein. This gene was also transcribed as a monocistronic mRNA from a promoter located in the manN-manO intergenic region. A search of current databases revealed the presence of IIAB(Man)L, ManM, ManN and ManO orthologues in Streptococcus mutans, Streptococcus pyogenes, Streptococcus pneumoniae and Enterococcus faecalis. This work has elucidated the molecular structure of the mannose PTS in streptococci and enterococci, and demonstrated the presence of a putative regulatory protein (ManO) within the man operon.