1H NMR studies of the mercuric ion binding protein MerP: sequential assignment, secondary structure and global fold of oxidized MerP.

The oxidized form of the mercuric ion binding protein MerP has been studied by two-dimensional NMR. MerP, which is a periplasmic water-soluble protein with 72 amino acids, is involved in the detoxification of mercuric ions in bacteria with resistance against mercury. The mercuric ions in the periplasmic space are first scavenged by the MerP protein, then transported into the cytoplasm by the membrane-bound transport protein MerT, and finally reduced to elementary (nontoxic) mercury by the enzyme mercuric reductase. In this work, the 1H NMR spectrum of oxidized MerP (closed disulfide bridge) has been assigned by using homonuclear 2D NMR techniques. The secondary structure and global fold have been inferred from the nuclear Overhauser effect (NOE) data. The secondary structure comprises four beta-strands and two alpha-helices, in the order beta 1 alpha 1 beta 2 beta 3 alpha 2 beta 4. The protein folds into an antiparallel beta-sheet, beta 2 beta 3 beta 1 beta 4, with the two antiparallel helices on one side of the sheet. The folding topology is similar to that of acylphosphatase, the activation domain of porcine pancreatic procarboxypeptidase B, the DNA-binding domain of bovine papillomavirus-1 E2 and the RNA-binding domains of the U1 snRNP A and hnRNP C proteins. However, there is no structural similarity between MerP and other bacterial periplasmic binding proteins.