Protein Folding in Vivo 
one-tenth of the amount of energy needed to syn- 
thesize the polypeptide. 
How general is the utilization of folding ma- 
chinery outside bacteria and mitochondria? We 
thought we might find a functionally similar com- 
ponent in the cytosol of higher organisms by look- 
ing first in the kingdom of archaebacteria. These 
organisms are evolutionarily distinct from bacte- 
ria and have recently been shown to contain com- 
ponents closely related to those in the cytosol of 
higher organisms. We transferred a thermophilic 
archaebacterium that grows normally at 75 °C to a 
near-lethal temperature of 88°C and found that a 
single major heat-shock protein was produced. 
This protein , already abundant at normal tempera- 
ture, is a double-ring complex that looks much 
like the complexes of mitochondria and bacteria, 
except that each ring contains nine members. 
The purified ring complex binds unfolded pro- 
teins and exhibits ATPase activity, consistent with 
the idea that it might function in the archaebac- 
teria similarly to the hsp60 and groEL complexes. 
Analysis of the protein's amino acid sequence 
demonstrated no significant relationship to ei- 
ther hsp60 or groEL, but rather a relationship to a 
protein called TCPl , an essential cytosol protein 
of higher organisms, whose function has been 
largely unknown but which has been implicated 
in assembly of the mitotic spindle apparatus. Us- 
ing a mutant yeast cell defective in TCPl , we are 
now investigating whether the protein has a spe- 
cialized function in assembly of the spindle or a 
more general function as a "folding machine" for 
the cytosol of higher organisms. 
MgATP 
groEL 
Model for folding of polypeptides by groEL. Polypeptide, shown in yellow, is bound by groEL and 
stabilized in "molten globule" conformation. When MgATP and groES are present, polypeptide 
undergoes conformational changes and is ultimately released, reaching a native, active 
conformation. 
Derived from Martin, f., Langer, T., Boteva, R., Scbramel, A., Horwich, A.L., and Hartl, F.-U. 
1991. Nature 352:36-42. 
190 
