During early development gene expression is controlled
principally at the translational level. Oocytes of the
surf clam Spisula solidissima contain large stockpiles
of maternal mRNAs that are translationally dormant or masked
until meiotic maturation. Activation of the oocyte by fertilization
leads to translational activation of the abundant cyclin
and ribonucleotide reductase mRNAs at a time when they
undergo cytoplasmic polyadenylation. In vitro unmasking
assays have defined U-rich regions located approximately
centrally in the 3′ UTRs of these mRNAs as translational
masking elements. A clam oocyte protein of 82 kDa, p82,
which selectively binds the masking elements, has been
proposed to act as a translational repressor. Importantly,
mRNA-specific unmasking in vitro occurs in the absence
of poly(A) extension. Here we show that clam p82 is related
to Xenopus CPEB, an RNA-binding protein that interacts
with the U-rich cytoplasmic polyadenylation elements (CPEs)
of maternal mRNAs and promotes their polyadenylation. Cloned
clam p82/CPEB shows extensive homology to Xenopus
CPEB and related polypeptides from mouse, goldfish, Drosophila
and Caenorhabditis elegans, particularly in their
RNA-binding C-terminal halves. Two short N-terminal islands
of sequence, of unknown function, are common to vertebrate
CPEBs and clam p82. p82 undergoes rapid phosphorylation
either directly or indirectly by cdc2 kinase after fertilization
in meiotically maturing clam oocytes, prior to its degradation
during the first cell cleavage. Phosphorylation precedes
and, according to inhibitor studies, may be required for
translational activation of maternal mRNA. These data suggest
that clam p82 may be a functional homolog of Xenopus
CPEB.