The family of heat shock proteins was initially characterized as a highly conserved battery of genes whose expression could be induced by heat shock. Many heat shock protein family members are now known to function constitutively as molecular chaperones, stabilizing and assisting in the trafficking of nascent peptides during normal growth. Under stressful conditions such as heat shock or hypoxia, increased expression of heat shock proteins protects the cell by stabilizing unfolded or misfolded peptides, giving the cell time to repair or re-synthesize damaged proteins.
Many heat shock proteins function together in co-chaperone complexes, such as Hsp70/Hsp40 (bacterial DnaK/DnaJ) that along with GrpE acts as an ATP-regulated shuttle complex for newly synthesized proteins. Many of these nascent peptides are delivered to Hsp90-containing complexes, which play a critical role in the stabilization and activation of key signaling kinases and hormone receptors. The Hsp60/Hsp10 complex (bacterial GroEL/GroES) forms an alternative protein folding mechanism in the mitochondria. Small heat shock proteins including Hsp27 and the crystallins form large oligomeric complexes that function to prevent protein aggregation.
