CRYSTAL STRUCTURE OF CheR, BACTERIAL CHEMOTAXIS RECEPTOR METHYLTRANSFERASE. Snezana Djordjevic and Ann Stock. Center for Advanced Biotechnology and Medicine, Department of Biochemistry, UMDNJ, Piscataway, New Jersey 08854, and Howard Hughes Medical Institute.

CheR is a methyltransferase which catalyzes the S-adenosyl-L-methionine-dependent transfer of methyl groups to specific glutamate side chains on a set of membrane chemoreceptor proteins that mediate bacterial chemotaxis. Methylation/demethylation of the receptors is involved in cellular adaptation to the concentrations of the chemoeffectors. An increase in attractant concentration in the medium results in an increase in the level of receptor methylation whereas a negative signal, such as removal of an attractant or an increase in repellent concentration is followed by a net decrease in receptor methylation.

The methyltransferase from Salmonella typhimurium, has been over-expressed in E. coli and purified to homogeneity. The enzyme was crystallized in the presence of the product of the methylation reaction, S-adenosyl-L-homocysteine. Multiple rounds of microseeding were required to obtain crystals suitable for X-ray diffraction studies. The crystal lattice belongs to a monoclinic space group P2₁ with unit cell dimensions of a = 55.0 Å, b = 48.0 Å, c = 63.2 Å and [[beta]] = 112.3deg.. There is one monomer of 283 residues per asymmetric unit and Matthew's value is 2.4 ų/Da. An X-ray diffraction native data set to 2 Å, with an R_sym of 4.9 %, has been collected on an R-axis II image plate detector. Initial phases were obtained to 3 Å using several mercurial and one platinum derivative crystals which gave an overall figure of merit of 0.71. An MIR electron density map was subjected to density modification procedure. This map contained interpretable elements of secondary structure and enabled us to build a partial model. Subsequent electron density maps were calculated by combining MIR and partial model phases. CheR is a mixed [[alpha]]/[[beta]] protein with two distinct domains. We have identified electron density corresponding to S-adenosyl-L-homocysteine associated with the larger domain.