Identification of a novel mRNA-associated protein in oocytes of Pleurodeles waltl and Xenopus laevis

Amphibian oocytes accumulate a large pool of mRNA molecules for future embr yonic development. Due to their association with specific proteins the stor ed maternal RNAs are translationally repressed. The identification of these RNA-binding proteins and the characterization of their functional domains may contribute to the understanding of the translational repression mechani sms and the subsequent activation processes during early embryogenesis. Her e we present the complete Pleurodeles cDNA sequence of a cytoplasmic protei n which is present in oocytes, eggs, and very early cleavage stage embryos but undetectable in postcleavage embryo and adult tissues. The predicted mo lecular mass of the protein is 55 kDa and the apparent molecular mass as de termined by SDS-PAGE, 68 kDa. The deduced amino acid sequence reveals proli ne- and serine-rich domains in the aminoterminal part as well as two RGG bo xes which represent characteristic motifs of several RNA-binding proteins. No distinct homologies to the consensus RNA recognition motif were found. T he 55-kDa protein was recovered in cytoplasmic ribonucleoprotein (RNP) part icles containing poly(A)(+) RNA. It was therefore termed RAP55 for mRNA-ass ociated protein of 55 kDa. However, a direct interaction of RAP55 with mRNA could not be demonstrated by UV-crosslinking experiments, indicating that it is bound to mRNP complexes via protein-protein interactions. RAP55 is ev olutionarily conserved since antibodies raised against a recombinant Pleuro deles RAP55 fragment recognize the protein from Pleurodeles and Xenopus. Th e expression pattern and intracellular distribution of RAP55 suggest that i t is part of those mRNP particles which are translationally repressed durin g oogenesis and become activated upon progesterone-induced oocyte maturatio n. (C) 1998 Academic Press.