In the transcriptionally inert maturing oocyte and early embryo, control of
gene expression is largely mediated by regulated changes in translational
activity of maternal mRNAs. Some mRNAs are activated in response to poly(A)
tail lengthening; in other cases activation results from de-repression of
the inactive or masked mRNA. The 3' UTR cis-acting elements that direct the
se changes are defined, principally in Xenopus and mouse, and the study of
their trans-acting binding factors is just beginning to shed light on the m
echanism and regulation of cytoplasmic polyadenylation and translational ma
sking. In the marine invertebrate, Spisula solidissima, the timing of activ
ation of three abundant mRNAs (encoding cyclin A and B and the small subuni
t of ribonucleotide reductase, RR) in fertilized oocytes correlates with th
eir cytoplasmic polyadenylation. However, in vitro, mRNA-specific unmasking
occurs in the absence of polyadenylation. In Walker et al. tin this issue)
we showed that p82, a protein defined as selectively binding the 3' UTR ma
sking elements, is a homolog of Xenopus CPEB (cytoplasmic polyadenylation e
lement binding protein). In functional studies reported here, the elements
that support polyadenylation in clam egg lysates include multiple U-rich CP
E-like motifs as well as the nuclear polyadenylation signal AAUAAA. This re
presents the first detailed analysis of invertebrate cis-acting cytoplasmic
polyadenylation signals. Polyadenylation activity correlates with p82 bind
ing in wild-type and CPE-mutant RR 3' UTR RNAs. Moreover, since anti-p82 an
tibodies specifically neutralize polyadenylation in egg lysates, we conclud
e that clam p82 is a functional homolog of Xenopus CPEB, and plays a positi
ve role in polyadenylation. Anti-p82 antibodies also result in specific tra
nslational activation of masked mRNAs in oocyte lysates, lending support to
our original model of clam p82 as a translational repressor. We propose th
erefore that clam p82/CPEB has dual functions in masking and cytoplasmic po
lyadenylation.