PROTEIN-SYNTHESIS INCREASES AFTER FERTILIZATION OF SEA-URCHIN EGGS INTHE ABSENCE OF AN INCREASE IN INTRACELLULAR PH

We have reevaluated the presumed requirement for an elevated intracell ular pH (pH(i)) in the acceleration of protein synthesis which follows fertilization of eggs of the sea urchin Lytechinus pictus. Zygotes we re transferred to sea water at a low pH (6.8) containing a permeant we ak acid at times ranging from 5 min to as early as 30 sec postinsemina tion, to reverse or prevent the rise in pH(i) that normally ensues upo n fertilization. Using the fluorescent pH probe 2',7'-bis(carboxyethyl )-5(6)-carboxyfluorescein (BCECF), we show that transfer of zygotes at 1 min or earlier after fertilization essentially blocks the rise in p H(i). Under these conditions, both the incorporation of radiolabeled l eucine into protein and the assembly of ribosomes into polysomes incre ase substantially (>50% of control values). We also assessed leucine i ncorporation during incubation of eggs and zygotes in sodium-free sea water or sea water containing amiloride, two additional treatments tha t block the pH(i) rise. In the presence of amiloride, leucine incorpor ation increased upon fertilization, whereas little or no increase was observed in sodium-free sea water. We provide evidence that the low ra tes of leucine incorporation in sodium-free sea water result from the tendency for this experimental condition to lower the pH(i) to values significantly lower than the pH(i) in unfertilized eggs. These finding s call into doubt the belief that the pH(i) rise at fertilization is a necessary prerequisite for the acceleration of bulk protein synthesis . These observations support the view that pH(i) is only one of severa l signals involved in the turn on of protein synthesis at the time of fertilization of sea urchin eggs. (C) 1995 Academic Press, Inc.