Mechanism of Release of Maternal Messenger RNA Induced by Fertilization in Sea Urchin Eggs

Abstract

An RNA which stimulates protein synthesis was found to be present in the 12, 000 × g precipitate from unfertilized sea urchin eggs when they were fractionated in the presence of Ca²⁺. Fertilization of the eggs caused translocation of the RNA from the precipitate to the microsomal fraction. A similar translocation was also observed on slight tryptic digestion of the homogenate of unfertilized eggs. In unfertilized eggs, the RNA was released from the precipitate by a brief treatment with trypsin [EC 3.4.4.4] and then polyribosomes seemed to be formed as a result of combination of the RNA and ribo-somes. The RNA resided in specific particles which sedimented in the range of (g-values between 8, 000 and 15, 000. The fraction contained also a trypsin-like protease whose activity manifested itself only after fertilization. Nucleic acid analysis revealed the particle to be different from ribosome. The particles were polydispcrse and sedimented at 300–400 S; they were found to be composed of vesicles in which electron dense particles of uniform size were enclosed. The release of the stored maternal messenger RNA unit from these specific particles by the activation of a trypsin-like protease seems to be a plausible cause of the immediate stimulation of protein synthesis induced by fertilization. The released maternal messenger RNA unit is believed to be a ribonucleo-protein particle which sediments in a polydisperse state at 20–60S. The activation of the protease mentioned above was temporary, suggesting that it is an event triggering the chain of processes of protein synthesis that follows.