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Fu, Xinmiao,Shi, Xiaodong,Yin, Linxiang,Liu, Jiafeng,Joo, Keehyoung,Lee, Jooyoung,Chang, Zengyi American Society for Biochemistry and Molecular Bi 2013 The Journal of biological chemistry Vol.288 No.17
<P>As ubiquitous molecular chaperones, small heat shock proteins (sHSPs) are crucial for protein homeostasis. It is not clear why sHSPs are able to bind a wide spectrum of non-native substrate proteins and how such binding is enhanced by heat shock. Here, by utilizing a genetically incorporated photo-cross-linker (<I>p</I>-benzoyl-<SMALL>l</SMALL>-phenylalanine), we systematically characterized the substrate-binding residues in IbpB (a sHSP from <I>Escherichia coli</I>) in living cells over a wide spectrum of temperatures (from 20 to 50 °C). A total of 20 and 48 residues were identified at normal and heat shock temperatures, respectively. They are not necessarily hydrophobic and can be classified into three types: types I and II were activated at low and normal temperatures, respectively, and type III mediated oligomerization at low temperature but switched to substrate binding at heat shock temperature. In addition, substrate binding of IbpB in living cells began at temperatures as low as 25 °C and was further enhanced upon temperature elevation. Together, these <I>in vivo</I> data provide novel structural insights into the wide substrate spectrum of sHSPs and suggest that sHSP is able to hierarchically activate its multi-type substrate-binding residues and thus act as a robust chaperone in cells under fluctuating growth conditions.</P>
Zhu, Jianhua,Fu, Xinmiao,Koo, Yoon Duck,Zhu, Jian-Kang,Jenney Jr., Francis E.,Adams, Michael W. W.,Zhu, Yanmei,Shi, Huazhong,Yun, Dae-Jin,Hasegawa, Paul M.,Bressan, Ray A. American Society for Microbiology 2007 Molecular and cellular biology Vol.27 No.14
<B>ABSTRACT</B><P>The myristoylated calcium sensor SOS3 and its interacting protein kinase, SOS2, play critical regulatory roles in salt tolerance. Mutations in either of these proteins render <I>Arabidopsis thaliana</I> plants hypersensitive to salt stress. We report here the isolation and characterization of a mutant called <I>enh1-1</I> that enhances the salt sensitivity of <I>sos3-1</I> and also causes increased salt sensitivity by itself. <I>ENH1</I> encodes a chloroplast-localized protein with a PDZ domain at the N-terminal region and a rubredoxin domain in the C-terminal part. Rubredoxins are known to be involved in the reduction of superoxide in some anaerobic bacteria. The <I>enh1-1</I> mutation causes enhanced accumulation of reactive oxygen species (ROS), particularly under salt stress. ROS also accumulate to higher levels in <I>sos2-1</I> but not in <I>sos3-1</I> mutants. The <I>enh1-1</I> mutation does not enhance <I>sos2-1</I> phenotypes. Also, <I>enh1-1</I> and <I>sos2-1</I> mutants, but not <I>sos3-1</I> mutants, show increased sensitivity to oxidative stress. These results indicate that ENH1 functions in the detoxification of reactive oxygen species resulting from salt stress by participating in a new salt tolerance pathway that may involve SOS2 but not SOS3.</P>