Regulation of Cell Polarity and RNA Localization in Vertebrate Oocytes - International Review of Cell and Molecular Biology - page 165

4.4. Oogenesis/oocyte maturation
4.4.1 acsl1b
In addition to regulating events after fertilization, recent analysis has iden-
tified functions for some localized mRNAs in the oocyte and egg itself.
Transcripts for acsl1b are localized by the late pathway and encode a long-
chain acyl-CoA synthetase ( King et al., 2005 ), an enzyme involved in lipid
metabolism and lipid modification of proteins. Surprisingly, depletion of
maternal acsl1b caused accelerated maturation of oocytes in response to pro-
gesterone ( Wang et al., 2012 ). Furthermore, Acsl1b was found to regulate
palmitoylation of Gnas (Galpha S), a component of a conserved G protein-
coupled receptor (GPCR) signaling pathway that maintains the arrest of
oocytes in G2 prior to maturation ( Wang et al., 2012 ). The role of Acsl1b
in later development was not examined, since the depleted oocytes were not
developmentally viable. The vegetal localization of acsl1b suggests some
animal-vegetal asymmetry to pathways regulating maturation, but the over-
all significance of these differences is unclear. It could reflect a general spatial
enrichment of GPCR signaling vegetally or a greater source of lipids stored
in the vegetal hemisphere.
4.4.2 rbpms
The RNA-binding protein Rbpms (Hermes) also functions in oocyte mat-
uration, although likely through different mechanisms. Both Rbpms RNA
and protein are vegetally localized in oocytes ( Zearfoss et al., 2004 ), and the
protein but not the RNA undergoes a drastic downregulation during oocyte
maturation. Interestingly, oocytes depleted of Rbpms protein undergo
accelerated maturation, suggesting that this protein might help maintain
G2 arrest ( Zearfoss et al., 2004 ). Consistent with this idea, Rbpms binds
to spdya ( speedy a ) mRNA, a Cdk2 regulator involved in meiotic progres-
sion, suggesting that Rbpms might repress Spdya translation ( Song et al.,
2007 ). Interestingly, Dazl (also and RRMprotein) and Pum2 regulate Spdya
translation similarly ( Ota et al., 2011 ), indicating that the germ plasm may
serve as a hub for cell cycle regulation, among other functions.
4.5. Structural roles for localized RNAs
4.5.1 xlsirts and vegt
In addition to facilitating localized protein synthesis, it is becoming apparent
that localized RNAs may also have functions at the RNA level. Some early
indications that vegetally localized RNAs might have protein-independent