CRYSTAL-STRUCTURES OF CHEY FROM THERMOTOGA-MARITIMA DO NOT SUPPORT CONVENTIONAL EXPLANATIONS FOR THE STRUCTURAL BASIS OF ENHANCED THERMOSTABILITY

The crystal structure of CheY protein from Thermotoga maritima has bee n determined in four crystal forms with and without Mg++ bound, at up to 1.9 Angstrom resolution. Structural comparisons with CheY from Esch erichia coli shows substantial similarity in their folds, with some co ncerted changes propagating away from the active site that suggest how phosphorylated CheY, a signal transduction protein in bacterial chemo taxis, is recognized by its targets. A highly conserved segment of the protein (the ''gamma-turn loop,'' residues 55-61), previously suggest ed to be a rigid recognition determinant, is for the first time seen i n two alternative conformations in the different crystal structures. A lthough CheY from Thermotoga has much higher thermal stability than it s mesophilic counterparts, comparison of structural features previousl y proposed to enhance thermostability such as hydrogen bonds, ion pair s, compactness, and hydrophobic surface burial would not suggest it to be so.