2.2 Chaperones help polypeptides to fold
We have seen how steric restrictions and energetic considerations specify preferred polypeptide conformations and ultimately determine a protein's three-dimensional structure. It is possible, of course, that there may be more than one energetically favourable conformation for a polypeptide. This is particularly true for large polypeptides. For a protein with a specific function in the cell, misfolding will affect its activity. Indeed, the misfolded protein may actually have some aberrant undesirable activity. To counter any damaging effect of misfolded proteins, mechanisms exist to facilitate correct folding or refolding of proteins and the removal/degradation of misfolded proteins.
Chaperones are proteins whose function it is to ensure correct folding of other proteins. There are two main families of chaperones in eukaryotic cells – hsp60 and hsp70 proteins – with different forms occurring in the cytosol, mitochondria and the endoplasmic reticulum, where they are responsible for refolding different proteins. The hsp nomenclature derives from the term ‘heat-shock protein’ which reflects the role of these proteins in the cellular response to elevated temperature. Incidences of protein misfolding increase when the cell experiences modest increases in temperature, and the cell responds by increasing expression of hsps to facilitate refolding.
Why might an increase in temperature encourage the misfolding of proteins?
Increased temperature would disrupt hydrogen bonds and increase molecular movements, thereby increasing the risk of incorrect folding.
Despite the presence of chaperones, misfolded proteins do occur and recognition and degradation of these aberrant molecules is essential for the health of the cell. Experimental evidence suggests that up to one-third of newly synthesised polypeptides are degraded rapidly. The protease responsible for degrading these proteins is the proteasome, a large multisubunit protein that can chop up protein substrates into small peptide fragments.