Control of Cell Polarity and Asymmetric Division in C. elegans - Current Topics in Developmental Biology - page 57

WRM-1 nuclear export is not known. The requirements of kinesin for
WRM-1 localization raised the model that microtubule-dependent
transport of WRM-1 toward the cell cortex removes it from the
perinuclear region, enhancing its nuclear export ( Sugioka et al., 2011 ).

3. ASYMMETRIC REGULATION OF TRANSCRIPTION
IN NUCLEI

In other organisms, b -catenin functions as a coactivator of transcription
factors of the TCF/LEF family. However, WRM-1 in C. elegans has a weak, if
any, activity to enhance transcription by POP-1/TCF ( Korswagen et al.,
2000; Natarajan et al., 2001 ). Instead, WRM-1 in the nucleus regulates
nuclear export of POP-1 through LIT-1 kinase that is homologous to
Nemo-like kinase in mammals ( Lo, Gay, Odom, Shi, & Lin, 2004 )
( Fig. 3.2 ). WRM-1 can bind to both LIT-1 and POP-1 ( Rocheleau et al.,
1999; Yang et al., 2011 ). Upon WRM-1 binding, LIT-1 is activated to
phosphorylate POP-1. Phosphorylated POP-1 binds to PAR-5/14-3-3
and is exported out of nucleus ( Lo et al., 2004 ). Similar to WRM-1, LIT-
1 is also enriched to the posterior nucleus after asymmetric divisions,
probably through its tight binding to WRM-1 ( Lo et al., 2004; Takeshita
& Sawa, 2005 ). Therefore, the phosphorylation and nuclear export of
POP-1 occurs asymmetrically in the posterior nucleus, creating high/low
POP-1 asymmetry (higher in the anterior nucleus) that is opposite to the
nuclear low/high WRM-1 asymmetry (higher in the posterior nucleus)
( Fig. 3.1 C) ( Lin, Thompson, & Priess, 1995 ).

Nuclear
export

POP

P

WRM

LIT
WRM

LIT

POP

LIT

WRM

POP

SYS POP

Transcription
activation

SYS

Figure 3.2 Regulation of transcription by POP-1. Free POP-1 binds either to SYS-1 to
activate transcription or to the WRM-1/LIT-1 complex to be exported out of nuclei.