JBC, Vol. 251, Issue 19, 5860-5865, Oct, 1976
Concanavalin A binding to membranes of the Golgi apparatus and resultant modification of galactosyltransferase activity
M. E. Young, M. A. Moscarello and J. R. Riordan
125I-Concanavalin A was found to bind to purified Golgi membranes in a
manner which was time-dependent, proportional to the amount of Golgi
protein, saturable, and inhibited by alpha-methyl-D-glucoside and unlabeled
concanavalin A. Approximately 2 nmol of 125I-concanavalin A were bound per
mg of membrane protein with a mean Ka of 0.2 x 10(7) M-1. After binding of
concanavalin A to the membranes, all unbound lectin was removed by repeated
washing prior to assay of galactosyltransferase activity to assure that
bulk concanavalin A would not bind to the sugar acceptor during the enzyme
assay. Therefore, the effects of the lectin were not due to altered
acceptor concentrations in the assay. As a result of concanavalin A binding
the activity of the galactosyltransferase of these membranes was enhanced
by 70 to 100%. The enhancement was abolished by treatment of the
concanavalin A-bound Golgi with alpha-methylglucoside. Kinetically this
stimulation was reflected as an altered Km for N-acetylglucosamine and
UDP-galactose. The data are suggestive that the stimulation of the enzyme
results indirectly from perturbation of the membranes on binding of the
lectin.