Mouse Hsp25, human Hsp27, and aß-crystallin are part of
a diverse family of small heat shock proteins (sHsps) which are
produced in all organisms. The overall homology between the different
sHsps is low and they are grouped together based on conserved sequences
in the C-terminal half of the protein and short conserved phenylalanine-rich
stretches near the N terminus. Mammalian sHsps are expressed constitutively
under physiological conditions but stress factors such as heat
shock induce a strong up-regulation of protein levels by 10-20-fold
to maximum concentrations of 0.1% of the cellular protein. They
function as chaperone-like proteins by binding unfolded polypeptides
and preventing uncontrolled protein aggregation. sHsps all share
the striking feature of forming high molecular weight oligomeric
complexes of variable size. The quaternary structure of sHsps is
essential for their function and regulation of activity but its
basic properties are still rather poorly understood. Data indicates
that Hsp25 is a dynamic tetramer of tetramers with a unique ability
to refold and reassemble into its active quaternary structure after
denaturation. Current studies demonstrate that Hsp25 helps facilitate
the glutathione-redox cycle by enhancing glutathione utilization
and maintaining the cellular glutathione pool in favor of the reduced
states.
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