Elevation of protein synthesis is a complex response to fertilisation.

Abstract

PROTEIN SYNTHESIS in unfertilised eggs of the sea urchin proceeds at a low rate. However, fertilisation triggers a series of reactions beginning with changes in intracellular cation concentrations within a few seconds of sperm entry and extending to a cascade of more complex events1. As these events are sequential in time, each might depend on the preceding event, or there might be a parallel series of independent pathways. The latter hypothesis was shown to be correct by Epel et al.2, who demonstrated that artificial activation with ammonia failed to initiate events such as sodium influx and the cortical reactions but did initiate a variety of others including potassium conductance, protein synthesis, DNA synthesis and mRNA polyadenylation. We have been studying the elevation of protein synthesis in detail. The rate of protein synthesis increases severalfold during the early cleavage stages3 and eventually reaches an absolute rate which is 100 times higher than that in the mature oocyte4. Kaumeyer et al.5 and Jenkins et al.6 have demonstrated that unfertilised eggs contain ‘masked’ messenger ribonucleoprotein particles which are not translatable in vitro unless modified or deproteinised. Most of the increase in the rate of protein synthesis following fertilisation is due to the recruitment of this oogenetic mRNA as a result of unmasking7. However, a significant portion of the rise also results from an increase in translational efficiency8. As defined by Palmiter9, translational efficiency is the number of completed polypeptides released per unit time per message. The change in efficiency following fertilisation of sea urchin eggs is reflected in an increase in the rate of ribosome movement (that is, the time required to complete an average size protein decreases after fertilisation) coupled to a concomitant increase in initiation rate. The net effect on protein synthesis of an increase in number of mRNAs available for translation and an increase in translational efficiency is multiplicative. We present evidence here that these two processes are under separate controls.