Archaea have a chaperone related to GroEL called a thermosome.

• The subunits of the thermosome are octamers rather than heptamers

• Domain folds are similar to those of GroEL
  • Sequence identity plus similarity is about 60%

  • Folds superpose with rmsd of 1.4 - 1.5 A for a-carbons

  Superposition of A Chains of theThermosome (blue) and GroEL (gold)

• The difference is almost entiely in the apical segment of the thermosome chain

The two major differences between the two chaperones are:

• The thermosome is almost spherical rather than cylindrical

• No equivalent of GroES is present; the thermosome forms its own lid

Closed Thermosome, Equatorial View	Closed Thermosome, Apical View (1a6d)
Open Thermosome, Equatorial View	Open Thermosome, Apical View (1a6e)

• As with GroEL, binding and refolding of misfolded proteins is coupled to hydrolysis of ATP

• The apical opening apparently enlarges with a motion similar to that of a camera lens

Eukaryotes likewise have chaperones, but very little structural or mechanistic information is available. The only chaperone that seems to have been characterized at all well is mammalian Hsp70:

Rat Hsp70 Protein Binding Domain (1ckr)	Human Hsp70 ATP Binding Domain (1s3x)

• Again, binding and refolding is driven by ATP hydrolysis

• Auxiliary proteins called Hip, Hop, and Bag switch on and off folding and release of the substrate

Thus, folding of proteins in the cell may be summarized as in this chart from an excellent review by Hartl et al. [Nature, 2011, 475, 324]: