The cyclic behavior of a cytoplasmic factor controlling nuclear membrane breakdown.

Abstract

Activity of a cytoplasmic factor (MPF), capable of inducing nuclear membrane breakdown (germinal vesicle breakdown) when injected into amphibian [frog] oocytes, was studied during the course of early cleavage in amphibian embryos. Mature egg cytoplasm apparently contained high levels of this activity, but this was quickly lost after fertilization or artificial activation. MPF activity later reappeared in the egg cytoplasm and started to cycle with time. The peak of embryonic MPF activity during each cycle coincided with the time the embryonic nuclei were entering the G2-M transition, i.e., mitosis. In colchicine-arrested embryos, this activity remained at an elevated level and no longer oscillated. Timing of appearance and disappearance of this activity appeared to be under control of the cytoplasm because such behavior was still observed in enucleated eggs. Continued protein synthesis in the embryo was required for the reappearance, but not for the disappearance, of this activity. MPF previously thought to be restricted to oocyte maturation may play a more general role in controlling nuclear membrane breakdown during mitosis and meiosis.