The variation of intermolecular contacts in helical aggregates of glutamine synthetase
Glutamine synthetase from E. coli is a large (MW 600,000) oligomeric enzyme composed of twelve identical polypeptide chains of MW 50,000. These twelve subunits are arranged at the vertices of two eclipsed hexagons to produce a structure with symmetry 622 or D6 as shown by electron microscopy and X-ray diffraction. The dodecameric molecule has a hexagonal profile of approximately 140 Å diameter and a thickness of approximately 90 Å. When treated with divalent cations such as Co++, Zn++, Ni++ or Cu++, glutamine synthetase molecules aggregate along their six-fold axes to form long strands and these strands will often wrap around one another to form cables (Fig. 1). The aggregation is completely reversed by the addition of excess Mn++ resulting in regeneration of full enzymatic activjjy. The most regular helical aggregates are formed in the presence of 10 mM Co++ at pH 7.0, and we have used these helical cables to study the three dimensional structure and symmetry of the glutamine synthetase molecule and the types of protein-protein interactions involved in the formation of glutamine synthetase cables.