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 47

Mutations in the NLS motif of LLA23 contain two defined regions, either
of which is necessary for partial nuclear targeting but both are required for
complete nuclear localization ( Wang et al., 2005 ). The putative NLS is also
conserved in other members of ASR proteins both in dicots and in mono-
cots ( Liu et al., 2010 ). The reported presence of putative bipartite NLSs in
a high proportion of plant b-ZIP proteins ( Raikhel, 1992 ; Varagona et al.,
1992 ) suggests that the bipartite structure may be the most prevalent NLS
configuration in plants across a spectrum of divergent nuclear proteins.
3.2.2. Intrinsically Unstructured Protein
The lily ASR protein is stable at 90 °C for 10 min ( Wang et al., 1996 ) and
the boiling-soluble feature is due to its flexible conformation in solution.
The protein contains relatively poor order-promoting amino acid residues
and enriched disorder-promoting amino acid residues that result in the
intrinsically unstructured conformation of the polypeptide ( Dai et al., 2011 ;
Goldgur et al., 2007 ). Intrinsically unstructured proteins have been found to
play crucial roles despite the lack of certain high-ordered structures ( Dyson
and Wright, 2005 ). For example, the unstructured form of ASR1 has
chaperone-like activity that helps stabilize macromolecules against dena-
turation caused by heat and freeze-thaw cycles ( Hsu et al., 2011 ; Konrad
and Bar-Zvi, 2008 ). The hydrophilic and boiling-soluble ASR protein may
function as an intracellular water retainer for preventing local dehydration
during stress ( Dai et al., 2011 ; Yang et al., 2005 ). Furthermore, the protec-
tive activity of ASR1 is synergistic with glycine-betaine, which accumulates
under unfavorable environmental conditions ( Konrad and Bar-Zvi, 2008 ).
In addition, ASR can fold into an ordered structure and form a homodimer
upon binding with zinc ions ( Goldgur et al., 2007 ).
Uversky et al. (2000) indicated that most natively unfolded proteins
have low-mean hydrophobicity, high net charge, and low molecular weight,
with less than 150 amino acid residues. For example, LEA proteins have a
relatively high composition of hydrophilic amino acids, most of which are
disordered under native conditions ( Battaglia et al., 2008 ; Mouillon et al.,
2008 ). ASR is another example that fits these criteria. As a hydrophilin,
Battaglia et al. (2008) classified ASR as a Group 7 LEA protein.
3.2.3. Cellular Localization
Subcellular fractionation experiments and immunodetection in tomato fruit
chromatin fractions suggest that tomato ASR1 is localized in the nucleus
( Iusem et al., 1993 ). Although ASR easily passes through the nuclear pore