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

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Abstract
Antibodies to the nucleoside 1,N6-ethenoadenosine were 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 1 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. The adenylylation sites are apparently on the periphery of the bilayered hexagonal disc, offset by 15 .+-. 10.degree. from the 2-fold axis of symmetry through a vertex of the hexagon and 20 .+-. 10 .ANG. from the plane between the layers of the disc.