Hsp27s belong to an abundant and ubiquitous family of small heat shock proteins (sHSP). It is an important HSP
found in both normal human cells and cancer cells. The
basic structure of most sHsps is a homologous and highly
conserved amino acid sequence, with an α-crystallin domain
at the C-terminus and the WD/EPF domain at the
less conserved N-terminus. This N-terminus is essential
for the development of high molecular oligomers (1, 2).
Hsp27-oligomers consist of stable dimers formed by as
many as 8-40 Hsp27 protein monomers (3). The
oligomerization status is connected with the chaperone
activity: aggregates of large oligomers have high
chaperone activity, whereas dimers have no chaperone
activity (4).
HSP27 is localized to the cytoplasm of unstressed cells
but can redistribute to the nucleus in response to stress,
where it may function to stabilize DNA and/or the
nuclear membrane. Other functions include chaperone
activity (as mentioned above), thermo tolerance in vivo,
inhibition of apoptosis, and signal transduction. Specifically,
in vitro, it acts as an ATP-independent chaperone by
inhibiting protein aggregation and by stabilizing partially
denatured proteins, which ensures refolding of the HSP70
complex. Hsp27 is also involved in the apoptotic signaling
pathway because it interferes with the activation of
cytochrome c/Apaf-1/dATP complex, thereby inhibiting the
activation of procaspase-9. It is also hypothesized that
hsp27 may serve some role in cross-bridge formation
between actin and myosin (5). And finally, Hsp27 is also
thought to be involved in the process of cell differentiation.
The up-regulation of Hsp27 correlates with the rate of
phosphorylation and with an increase of large oligomers. It
is possible that Hsp27 may play a crucial role in termination
of growth (6).