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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 A 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 chaperonin-assisted protein folding reaction.