Regulation of Cell Polarity and RNA Localization in Vertebrate Oocytes - International Review of Cell and Molecular Biology - page 164

4.3.3 trim36
Depletion of maternal trim36 also abrogated microtubule assembly, cortical
rotation, and subsequent axis formation ( Cuykendall and Houston, 2009 ).
Trim36 is a RING finger ubiquitin ligase of the tripartite-motif-containing
family and belongs to a subfamily that can associate with microtubules ( Short
and Cox, 2006 ). Rescue experiments using full-length or mutant constructs
showed that this ubiquitin ligase activity was essential to rescue microtubule
assembly ( Cuykendall and Houston, 2009 ). This was the first suggestion that
regulation of ubiquitylation of microtuble-associated proteins might be
required for microtubule assembly during cortical rotation. In cultured cells,
ubiquitin colocalizes with microtubules (tubulin subunits are not directly
ubiquitylated; Murti et al., 1988 ), although the overall role of ubiquitylation
in microtubule function is little understood. Recently, it has been discov-
ered that Dnd1 likely regulates translation of Trim36 (J. Yang, personal
communication), suggesting that localized RNAs may engage in compart-
mentalized regulatory interactions with each other.

4.3.4 syntabulin
Teleost fish, such as medaka and zebrafish, also depend on microtubule-
based transport of dorsal determinants in axis formation ( Jesuthasan and
St¨hle, 1997; Tran et al., 2012 ) but do not likely undergo relative
cortical-cytoplasmic displacement (i.e., cortical rotation; Clavert, 1962;
Ho, 1992b ). The maternal zebrafish mutant, tokkaebi ( tkk) , was isolated in
a screen for defective neural development ( Nojima et al., 2004 ), with tkk
mutants having a ventralized appearance and lacking expression of dorsal
beta-catenin target genes ( Nojima et al., 2004 ). The affected gene was iden-
tified as syntabulin ( sybu ), which encodes a Kif5b linker protein ( Nojima
et al., 2010 ) implicated in presynaptic protein localization and mitochondrial
localization in neurons ( Frederick and Shaw, 2007; Su et al., 2004 ).
Sybu is proposed to transport putative dorsal determinants, since its
expression and localization in zygotes is consistent with the localization of
these putative factors, and since vegetal microtubule assembly is not affected
in tkk mutants ( Nojima et al., 2010 ). However, since maternal homozygous
tkk fish would lack Sybu throughout oogenesis, Sybu may have an
uncharacterized earlier role in localizing dorsal determinants to the mito-
chondrial cloud, vegetal cortex, or elsewhere. Regardless, the identification
of Sybu-binding proteins

should help identify new components in

axis formation.