New Insights into Desiccation-Associated Gene Regulation by Lilium longiflorum ASR during Pollen Maturation and in Transgenic Arabidopsis - International Review of Cell and Molecular Biology - page 49

drought response element binding (DREB) proteins ( Saumonneau et al.,
2008 ). Shkolnik and Bar-Zvi (2008) have further demonstrated the tomato
ASR1-bound coupling element 1 (CE1) of ABI4 promoter competing
with ABI4, which produced an ABA insensitive phenotype of ASR1-over-
expressing plants in Arabidopsis .
In addition to nuclear localization, substantial amounts of ASR still
remain in the cytoplasm. Kalifa et al. (2004a) indicated that two-thirds
of the total ASR1 protein is detected in the cytoplasm using indirect
immunofluorescence analysis. In pollen grains, ASR is located both in the
cytoplasm and in the nucleus of generative and vegetative cells, which has
been confirmed using the immunogold labeling of LLA23 ( Huang et al., 2000 ;
Wang et al., 2005 ). Embryogenic calli of transgenic rice carrying an
ASR5-GFP fusion revealed that ASR5 was localized in both the nucleus
and cytoplasm ( Arenhart et al., 2012 ). Aside from ASR, several nuclear pro-
teins are also found in the cytoplasm ( Houde et al., 1995 ; Yoneda, 2000 ).
The ASR protein remaining in the cytoplasm is quite intriguing. Although
the actual mechanism by which the protein is retained in the cytoplasm is
unknown, it may perhaps involve either the masking of NLS or protein-
protein interactions that serve to anchor the regulatory protein in the cyto-
plasmic compartment ( Beaudoin and Labbé, 2006 ).
3.2.4. Nucleocytoplasmic Transport of ASR
In addition to the small molecules that diffuse passively through nuclear
pores along their concentration gradient, large molecules can be actively
transported using the NLS sequence recognized by receptors that dock
with their cargo at the nuclear pore ( Jans et al., 2000 ; Yoneda, 2000 ). The
proteins in the nucleus or in the cytoplasm are at least partly regulated
by dehydration. Dehydration induces the translocation of LLA23 from
the cytoplasm into the nucleus during pollen maturation ( Yang et al.,
2008 ). Multiple nucleoplasmic transport pathways have been suggested
( Yoneda, 2000 ). The nucleocytoplasmic transport of proteins is regulated by
the masking/unmasking reactions of NLSs through their binding proteins
( Jans et al., 2000 ; Yoneda, 2000 ). For example, a transcription factor, Cuf1,
is localized in the cytoplasm via the masking of its NLS by intramolecular
conformational changes in the induced metalation of Cuf1 in the presence
of elevated copper concentrations ( Beaudoin and Labbé, 2006 ). In some
cases, the heterodimerization or homodimerization of proteins is involved
in the regulation of the nuclear import of proteins. The DNA-bound ASR1
is folded in a dimeric protein structure ( Goldgur et al., 2007 ); however,