Early genetic studies identified the Escherichia coli groES and groEL genes
because mutations in them blocked the growth of bacteriophages lambda and
T4. Subsequent genetic and biochemical analyses have shown that GroES and G
roEL constitute a chaperonin machine, absolutely essential for E, coli grow
th, because it is needed for the correct folding of many of its proteins. I
n spite of very little sequence identity to GroES, the bacteriophage T4-enc
oded Gp31 protein and the bacteriophage RB49-encoded CocO protein are bona
fide GroEL cochaperonins, even capable of substituting for GroES in E. coli
growth. A major functional distinction is that only Gp31 and CocO can assi
st GroEL in the correct folding of Gp23, the major bacteriophage capsid pro
tein. Conserved structural features between CocO and Gp31, which are absent
from GroES, highlight their potential importance in specific cochaperonin
function.