Chaperonins are universally conserved molecular machines that facilitate the 
proper -folding of nascent and partially folded polypeptides into their 
respective three-dimensional structures. These multimeric protein complexes 
utilize the energy derived from ATP hydrolysis to fuel a protein-folding 
mechanism that consists of multiple rounds of substrate binding, encapsulation, 
and eventual expulsion back into the cytosolic environment. In this portion of 
the chapter, the structure and function of group I and group II chaperonins are 
discussed. Furthermore, the general mechanism of chaperonin-mediated protein 
folding is addressed in addition to illustrating how viral phages such as 
Lambda, T4, and RB49 exploit the host machinery for the proper folding of viral 
gene products. Lastly, the phiEL chaperonin from phage EL is revealed to be the 
first virally encoded chaperonin and is proposed to function independently of 
the host chaperonin machinery. The molecular architecture of the phiEL 
chaperonin, coupled with its unique functional abilities, renders its 
characterization a challenge and further highlights its novelty as a potentially 
whole new class of chaperonins.