Regulation of Cell Polarity and RNA Localization in Vertebrate Oocytes - International Review of Cell and Molecular Biology - page 151

Deletion mapping and site-directed mutagenesis have defined sequence
motifs in the nos1 3 0 -UTR required for various aspects of its localization.
nos1 has a 240-nt element required for MCLS adjacent to the coding region
( Chang et al., 2004; Zhou and King, 1996 ). The 3 0 -end of the RNA, which
is rather short, contains a germinal granule localization element ( Kloc et al.,
2000, 2002a ). Repeats of UGCAC are clustered in the MCLS and are nec-
essary but not sufficient for MCLC ( Betley et al., 2002 ). Igf2bp3 binds nos1 ,
but this binding does not require the UGCAC motifs, which are similar to
the consensus UUCAC Igf2bp3-binding motif ( Chang et al., 2004 ). Indeed,
UV-cross-linking studies showed that the nos1 LE and the gdf1 LE bind
essentially the same set of proteins, including Igf2bp3 and Ptbp1 ( Chang
et al., 2004 ). Additionally, other germ plasm RNAs, such as xlsirts and dazl ,
behave nearly identically to nos1 in localization experiments yet lack similar
motifs in the MCLS ( Betley et al., 2002 ). These studies underscore the over-
all complexity of early pathway RNA localization mechanisms in oocytes.
3.2.2 Localization of early pathway transcripts to the germinal granules
Ultrastructurally, germ plasm within the mitochondrial cloud has several
components, including mitochondria, other organelles, and germinal gran-
ule material ( Houston and King, 2000b ). In a technical tour-de-force, Kloc
et al. (2000, 2002a,b) used EM- in situ hybridization to characterize the sub-
localization of endogenous early pathway mRNAs within the germ plasm.
Also, a minimal cis -element required for localization to the germinal granules
was defined. These studies showed that nos1 is likely a marker for germinal
granule material at all stages examined, localizing to nuage material in
prediplotene cyst oocytes, condensing GFM in the mitochondrial and
denser germinal granules in mature oocytes and embryos ( Kloc et al.,
2000, 2002a ). The germinal granule localization element is adjacent to
the MCLS and is necessary and sufficient for granule localization of exoge-
nous transcripts, although granule targeting requires the presence of the
MCLS ( Kloc et al., 2000, 2002a ). Specific RNA sequence motifs for germi-
nal granule localization have not been identified. Of several early pathway
RNAs examined, only nos1 , xpat , and ddx25 ( deadsouth ) associated with
granules during oogenesis ( Kloc et al., 2002a,b ), the remainder were
restricted to the germ plasm but localized to matrix surrounding the gran-
ules. Only nos1 showed localization to granules during embryogenesis, an
association that was lost prior to the onset of PGC migration.
The significance of these localizations is not clear, other than to suggest
an integral role for nos1 in germinal granule biology. The localization