In response to adverse changes in their environment,
cells from many organisms increase the expression of a class of
proteins referred to as heat shock or stress proteins. One class
of stress proteins, termed the Hsp70 family, is comprised of multiple
members, all of which bind ATP in vitro, but which are localized
within different intracellular compartments. These include: i)
Hsc70 (or constitutive form) present within the cytosol/nucleus;
ii) Hsp70 (inducible form) present within the cytosol/nucleus/nucleolus;
iii) the constitutive glucose-regulated 78 kDa (or BiP) protein
present within the lumen of the endoplasmic reticulum; and iv)
the constitutive glucose regulated 75 kDa protein present within
the mitochondrial matrix. Members of the Hsp70 family are thought
to function as molecular chaperones, assisting in the folding of
other proteins in various intracellular compartments. Grp75 is
localized in the mitochondrial matrix, where, in concert with Hsp60,
is thought to participate in the re-folding of proteins translocated
into this organelle. Like its E. coli homolog DnaK, Grp75 possesses
a cation-dependent ATPase activity thought to be central to its
function as a chaperone.
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