THE 3-DIMENSIONAL STRUCTURE OF THE LIGAND-BINDING DOMAIN OF A WILD-TYPE BACTERIAL CHEMOTAXIS RECEPTOR - STRUCTURAL COMPARISON TO THE CROSS-LINKED MUTANT FORMS AND CONFORMATIONAL-CHANGES UPON LIGAND-BINDING

The three-dimensional structures of the ligand-binding domain of the w ild-type Salmonella typhimurium aspartate receptor have been determine d in the absence (apo) and presence of bound aspartate (complex) and c ompared to a cross-linked mutant containing a cysteine at position 36 which does not change signaling behavior of the intact receptor. The s tructures of the wild-type forms were determined in order to assess th e effects of cross-linking on the structure and its influence on confo rmational changes upon ligand binding. As in the case of the cross-lin ked mutant receptor, the non-cross-linked ligand-binding domain is dim eric and is composed of 4-alpha-helical bundle monomer subunits relate d by a crystallographic 2-fold axis in the unbound form and by a non-c rystallographic axis in the aspartate-bound form. A comparative study between the non-cross-linked and cross-linked structures has led to th e following observations: 1) The long N-terminal helices of the indivi dual subunits in the cross-linked structures are bent toward each othe r to accommodate the disulfide bond. 2) The rest of the subunit confor mation is very similar to that of the wild-type. 3) The intersubunit a ngle of the cross-linked apo structure is larger by about 13 degrees w hen compared to the wild-type apo structure. 4) The nature and magnitu de of the aspartate-induced conformational changes in the non-crosslin ked wild-type structures are very similar to those of the cross-linked structures.