Protein Folding In Vivo 
the folding machine might self-assemble to pro- 
duce a new complex. We recently tested this pos- 
sibility and found that hsp60 subunits could not 
self-assemble inside mitochondria. Assembly re- 
quired the presence of preexistent functional 
hsp60 complex. 
How did the mitochondrial hsp60 complex get 
assembled in the first place? This question is an- 
swerable in terms of the origin of mitochondria. 
It seems likely that when the original mitochon- 
drion was founded as an ingested bacterial cell, it 
already contained an hsp60-related protein. The 
question, then, really becomes, How was the 
hsp60-related component in the bacterial pro- 
genitor assembled for the first time? It seems pos- 
sible that assembly might have occurred sponta- 
neously, inaugurating a perpetual catalyzed 
assembly reaction. Alternatively, initial assembly 
might have occurred with the help of some other 
component — another protein, or perhaps a nu- 
cleic acid. 
Thus we have shown that imported mitochon- 
drial proteins do not fold and assemble on their 
own but, rather, require assistance from a molecu- 
lar machinery. This may also prove to be the case 
for proteins of other cellular compartments. In 
general, in the living cell, newly made and newly 
translocated proteins may achieve their active 
conformations using machines that function like 
the hsp60 complex. 
202 
