Structural adaptations in the specialized bacteriophage T4 co- chaperonin Gp31 expand the size of the anfinsen cage

The Gp31 protein from bacteriophage T4 functionally substitutes for the bacterial co-chaperonin GroES in assisted protein folding reactions both in vitro and in vivo. But Gp31 is required for the folding and/or assembly of the T4 major capsid protein Gp23, and this requirement cannot be satisfied by GroES. The 2.3 Å crystal structure of Gp31 shows that its tertiary and quaternary structures are similar to those of GroES despite the existence of only 14% sequence identity between the two proteins. However, Gp31 shows a series of structural adaptations which will increase the size and the hydrophilicity of the 'Anfinsen cage,' the enclosed cavity within the GroEL/ GroES complex that is the location of the chaperoninassisted protein folding reaction.