Localization of the site of adenylylation of glutamine synthetase by electron microscopy of an enzyme-antibody complex

Abstract

Antibodies to the nucleosidel,N(6)-ethenoadenosine have been used to localize the site of adenylylation of the glutamine synthetase [L-glutamate:ammonia ligase (ADP-forming), EC 6.3.1.2] of Escherichia coli. Antibodies were induced in rabbits by injection of a bovine albumin-ethenoadenosine conjugate. The resulting antisera strongly bound ethenoadenosine, its 5'-nucleotide, or protein conjugates of the nucleoside; little or no crossreaction was seen to adenosine, AMP, or the protein carrier. Ethenoadenylylated glutamine synthetase was prepared by modification of the enzyme by the E. coli adenylyltransferase, using etheno-ATP as a substrate. The ethenoadenylylated glutamine synthetase was precipitated by antibodies to ethenoadenosine in conjunction with goat anti-rabbit gamma globulin. Electron micrographs of reaction mixtures of ethenoadenylylated glutamine synthetase and anti-ethenoadenosine showed individual enzyme molecules complexed with one or more antibodies and pairs of enzyme molecules crosslinked by a single antibody. The approximate site of adenylylation was located from the apparent area of contact between enzyme and antibody. We conclude that the adenylylation sites are on the periphery of the bilayered hexagonal disc, offset by 15 +/- 10 degrees from the 2-fold axis of symmetry through a vertex of the hexagon and 20 +/- 10 A from the plane between the layers of the disc.