Two related localized mRNAs from Xenopus laevis encode ubiquitin-like fusion proteins

The uneven distribution of maternal mRNAs in unfertilized eggs and the unequal inheritance of these molecules by dividing blastomeres may be one mechanism for determining cell fate during embryogenesis. Complementary DNA (cDNA) clones corresponding to maternal mRNAs localized to specific regions of the Xenopus laevis egg have been previously identified and cloned [Rebagliati et al., Cell 42(1985) 769-777]. The maternal mRNA, An1, was originally identified as being localized to the animal hemisphere of X. laevis eggs and early embryos. We describe here the two proteins encoded by two An1 mRNA isoforms which we designate An1a and An1b. These mRNAs are both approximately 3.0 kb long and are concentrated in the animal hemisphere of unfertilized eggs. The predicted amino acid (aa) sequences encoded by An1a and An1b correspond to 76.9 and 78.6 kDa, respectively, and are 88% identical. Both proteins contain a single N-terminal ubiquitin (Ub)-like domain (50% identical to X. laevis Ub) and a putative Zn(2+)-binding region near the C terminus. Unlike Ub polyproteins and most Ub fusion proteins, the N-terminal Ub-like domain found in the An1 proteins does not undergo proteolytic processing. In contrast to earlier studies showing that the An1 mRNA represents a strictly maternal transcript, we report that both related An1 transcripts are found in later embryonic stages and in all adult tissues tested.