Pseudo-T-even bacteriophage RB49 encodes CocO, a cochaperonin for GroEL, which can substitute for Escherichia coli's GroES and bacteriophage T4's Gp31

Bacteriophage T4-encoded Gp31 is a functional ortholog of the Escherichia c oli GroES cochaperonin protein. Both of these proteins form transient, prod uctive complexes with the GroEL chaperonin, required for protein folding an d other related functions in the cell, However, Gp31 is specifically requir ed, in conjunction with GroEL, for the correct folding of Gp23, the major c apsid protein of T4, To better understand the interaction between GroEL and its cochaperonin cognates, we determined whether the so-called "pseudo-T-e ven bacteriophages" are dependent on host GroEL function and whether they a lso encode their own cochaperonin, Here, we report the isolation of an alle le-specific mutation of bacteriophage RB49, called epsilon 22, which permit s growth on the E, coli groEL44 mutant hut not on the isogenic wild type ho st. RB49 epsilon 22 was used in marker rescue experiments to identify the c orresponding wild type gene, which me have named cocO (cochaperonin cognate ). CocO has extremely limited identity to GroES but is 34% identical and 55 % similar at the protein sequence level to T4 Gp31, sharing all of the stru ctural features of Gp31 that distinguish it from GroES. CocO can substitute for Gp31 in T4 growth and also suppresses the temperature-sensitive phenot ype of the E. coli groES42 mutant. CocO's predicted mobile loop is one resi due longer than that of Gp31, with the epsilon 22 mutation resulting in a Q 36R substitution in this extra residue, Both the CocO wild type and epsilon 22 proteins have been purified and shown in vitro to assist GroEL in the r efolding of denatured citrate synthase.