195cess, including GV migration to the animal pole and translation of RNAs promoting
meiotic progression (Fig. 5.2c). Before the oocyte matures, animally localized
RNAs like cycB are translationally repressed. CycB protein is required during
oocyte maturation, along with Cdc2 kinase, as part of the maturation-promoting
factor (MPF) that drives meiotic progression (Kondo et al. 1997 ; Masui 1972 ;
Nagahama and Yamashita 2008 ). Thus, cycB mRNA localization to the animal pole
is important for CycB localized translation and activity, which allows, in conjunc-
tion with other factors, oocyte meiotic progression and competence.
Interestingly, cycB localization is regulated both by its 3′UTR as well as
sequences within its coding region (Yasuda et al. 2010 ). In a transgenic reporter
assay, the cycB 3′UTR is sufficient for localization to the animal pole in stage III
zebrafish oocytes. However, localization is transient, and by stage IV of oogenesis,
the reporter RNA is no longer localized. It was determined that a 9 bp sequence
within the ORF is required for its stable cortical localization. Importantly this
sequence is conserved in vertebrate cycB genes suggesting its conserved function
(Yasuda et al. 2013 ). This 9 bp sequence also functions in mediating the precise
timing of cycB translation, since when it is mutated, the reporter is translated pre-
maturely following oocyte maturation induction (Yasuda et al. 2013 ).
In frogs, the timing of cycB translation is regulated by two RNA-binding pro-
teins, the cytoplasmic polyadenylation element-binding protein (CPEB) and Pumilio
1 (Pum1) (Mendez and Richter 2001 ; Nakahata et al. 2001 ; Pique et al. 2008 ).
While CPEB-dependent polyadenylation leads to recruitment of cycB RNA into
polysomes, Pum1 regulates the timing of its translation. Thus, RNAs containing
CPEB are translated at different times during maturation. Pum1 functions in the
assembly of cycB RNA granules that are translationally silent before maturation. In
zebrafish and mouse oocytes, cycB RNA granules disassemble after oocyte matura-
tion induction, which coincides with the onset of CycB protein synthesis. Failure of
Pum1 binding to the cycB 3 ′UTR causes precocious translation of CycB protein in
maturing oocytes. In addition to Pum1 binding, actin filaments are also required for
cycB translational timing during maturation. Actin filaments regulate RNA granule
assembly and disassembly dynamics and in doing so affect the timing of cycB RNA
translation (Kotani et al. 2013 ; Yasuda et al. 2010 ).
5.7 The Hydrogel and the Intrinsically Disordered Proteins
Theory
The Bb is a specialized massive mRNP granule. Many different types of mRNP
granules exist across cell types and species. These include P granules in C. elegans
oocytes and embryos, germ granules in zebrafish and mice germ cells, the chroma-
toid body in mouse spermatids, polar granules in Drosophila oocytes, neuronal
granules, stress granules, and P bodies. A common theme to all mRNP granule types
is the tight clustering of mRNA and proteins, which is distinct from other soluble
5 Localization in Oogenesis of Maternal Regulators of Embryonic Development