189localization of injected Vg1 RNA. These motifs are the sequences UUUCU (VM1)
and UUCAC (E2) that are necessary also for localization of RNAs through the early
pathway. The clustering of E2 and/or VM1 motifs might be important for LE recog-
nition (Betley et al. 2002 ; Choo et al. 2005 ; King et al. 2005 ).
RNA localization ultimately relies on RNA-binding proteins that bind to the LE
and target it to the vegetal oocyte cortex. Studies in frog have identified a number of
proteins that bind the Vg1 RNA. Among these Igf2bp3 (also called Vg1RBP and
Vera) and Ptbp1 (also called VgRBP60 and hnRNPI) were shown to bind the Vg1
LE and function in Vg1 localization. Whereas Ptbp1 binds to Vg1 through the VM1
motifs of the LE, Igf2bp3 does so through the E2 sites (Lewis et al. 2004 ). Mutations
in the Vg1 LE affect Igf2bp3 and Ptbp1 binding and disrupt Vg1 localization
(Deshler et al. 1997 , 1998 ; Havin et al. 1998 ).
The proteins that bind and localize RNAs first associate with the RNA either in the
nucleus or the cytoplasm to form the RNP transport complex. Igf2bp3 and Ptbp1 asso-
ciate with Vg1 and VegT RNAs in the nucleus, and the RNP complex is then exported
to the cytoplasm (Kress et al. 2004 ). The RNA-binding protein Staufen, first identified
in flies to play a role in RNA localization (Brendza et al. 2000 , 2002 ), was shown in
frogs to bind Vg1 and Kinesin I in the cytoplasm (Yoon and Mowry 2004 ). Moreover,
expression of mutated forms of Staufen, lacking two out of five RNA-binding domains
and the tubulin-binding domain, disrupts Vg1 localization and suggests that Staufen
may be a functional link between the RNA and the MT transport machinery required
for localizing RNA (Micklem et al. 2000 ; Yoon and Mowry 2004 ).
Little is known in zebrafish about the mechanisms and RNA-binding proteins
(RBPs) acting in localizing RNAs through the late pathway. As in frogs, Staufen
and Ptbp1are expressed during oogenesis (Bateman et al. 2004 ; Mei et al. 2009 ).
Staufen depletion in embryos causes defects in PGC migration and survival
(Ramasamy et al. 2006 ; Yoon and Mowry 2004 ); however, its function in RNA
localization in oogenesis has not been tested. Ptbp1a/Brom bones functions in egg
activation, where it is required for IP3-mediated Ca2+ release in egg activation. A
second copy of the ptbp1 gene, ptbp1b, was found in zebrafish that is also expressed
during oogenesis, suggesting the possibility that these genes act redundantly in
RNA localization (Mei et al. 2009 ). Further studies are required to address specifi-
cally the function of these RBPs in RNA localization during zebrafish oogenesis.
The current understanding of the RNA localization process in Xenopus oocytes
suggests that early and late pathways may not represent two independent events, but
rather a continuum mechanism of RNA localization. Interestingly, when RNAs that
localize through the early pathway are injected into stage III oocytes in Xenopus,
they can still localize to the vegetal cortex (Choo et al. 2005 ; Claussen et al. 2004 ;
Zhou and King 1996a). Moreover, the late pathway Vg1-LE when injected into
stage I oocytes can localize to the Bb (Choo et al. 2005 ). Importantly, these RNAs
require the same motifs in their 3′UTRs for early and late pathway localization, sug-
gesting that the RNA-protein machinery functioning in the late pathway may be
similar to that used in the early pathway (Choo et al. 2005 ; Claussen et al. 2004 ).
Technologies like the MS2 RNA-labeling system are helping to elucidate the
real-time dynamics of RNA localization in different contexts. The MS2 system
5 Localization in Oogenesis of Maternal Regulators of Embryonic Development