Effect of reduced protein synthesis on the cell cycle in sea urchin embryos

We have reinvestigated the existence of cyclical fluctuations of protein synthesis and have examined the effects of reducing it in early embryos of the purple sea urchin, Strongylocentrotus purpuratus. The results show that protein synthesis increases linearly during the first 45-60 minutes after fertilization, then transiently decreases during mitosis, and rises again at first cleavage. Reducing protein synthesis of embryos to 35% its normal value only slightly affects the rate of progression through the cell cycle. It is also shown that the observed retardations of the cell cycle, under depressed protein synthesis, are attributable (by 80%) to a lengthening of the premitotic phase but also, to a lesser extent (20%), to a lengthening of the mitotic phase itself. These results suggest that mitotic proteins, in sea urchin embryos, are stable and little affected by an imposed decrease of protein synthesis during their accumulation phase. This analysis supports the view that specific mechanisms, other than decreased protein synthesis, need be turned on only at appropriate times during the cell cycle in order to explain the destruction or deactivation of mitotic proteins. Finally, a one-dimensional SDS-PAGE analysis of synthesized proteins, labeled with 35S-methionine, reveals the presence of a 50-kDa cyclin showing the expected characteristics of mitotic proteins deduced from our results.