Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

High Temperature Stress
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tifs and appear to recruit other hsps to help repair damaged proteins. In yeast,
Hsp104 can reverse the effect of high temperature on protein structure. Yeast
Hsp104 acts with other hsps (Hsp40 and Hsp70) to undo damage to proteins
caused by heat (Glover and Lindquist, 1998).
Hsp90 – In animals Hsp90 is best known as a component of a complex that helps fold
the glucocoticoid receptor. In plants, Hsp90 participates in a similar multiprotein
complex that also includes Hsp70 and a small acidic protein known as p23 (Pratt et
al., 2001). Hsp90 complexes appear to be involved not only in protein folding, but
also protein transport along microtubules inside the cell, especially transport of
proteins into peroxisomes (Pratt et al., 2001). Hsp90 is essential for growth and is
induced by low temperature (Krishna et al., 1995) in addition to being prominently
expressed at high temperature. Hsp90 has been called a “capacitor” for genetic
variation. In this view, Hsp90 can cover up developmental abnormalities, but when
Hsp90 levels are reduced, these abnormalities get expressed and this, in essence,
creates a burst of evolution in response to a stressful environment (Queitsch et al.,
2002).
Hsp 70 – Like Hsp90, Hsp70 is required for plant growth and is essential for processes
such as protein import into chloroplasts and mitochondria (Marshall et al., 1990).
Hsp70 proteins appear to be involved in virus infectivity and perhaps in allowing
viruses to move between cells of plants through plasmadesmata (Alzhanova et al.,
2001).
Hsp60 – Rubisco folding requires a “rubisco binding protein” that appears to be a
member of the Hsp60 family. As many as 14 Hsp60 protein chains will aggregate to
make a functional complex to fold rubisco (Roy and Andrews, 2000). Hsp60s are
essential for folding of many proteins in plants and it is presumed are needed for
refolding when proteins are denatured by heat.
smhsp (mol wt 17 to 28 kDa) – The family of small heat shock proteins is much bigger in
plants than other organisms such as fungi or animals (Vierling, 1991). It has been
hypothesized that the rapid divergence of small heat shock proteins in plants into
five or six families may have been an important step in the evolution of plants onto
land required to allow plants to cope with the large temperature fluctuations that
occur in air with its low heat capacity relative to water (Waters, 1995, 2003). The
families of small heat shock proteins in plants are located in compartments, one in
the chloroplast, one in the endoplasmic reticulum, and two families in the cytosol.
It was recently shown that there are two small hsp families in the mitochondrion
(Scharf et al., 2001). The plant small hsps appear to play a role in thermotolerance
of a number of processes (Basha et al., 2004). Modulation of thermotolerance in
carrot cell cultures has been reported by modulating the abundance of a small hsp
(Malik et al., 1999). The chloroplast small heat shock proteins have been associated
with increased thermotolerance (Wang and Luthe, 2003) and the mechanism
has been suggested to be protection of photosystem II (Heckathorn et al., 1998,