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Unravelling the chaperonin‐assisted folding of the bacteriophage T4 major capsid protein

Folding of bacteriophage T4 major capsid protein, gp23, is essential to the T4 morphogenesis, and is strictly dependent on Escherichia coli chaperonin GroEL in conjunction with the bacteriophage T4‐ encoded co‐chaperonin, gp31. Gp31 is similar in overall structure to the host homologue GroES, which is unable to assist the GroEL‐mediated folding of gp23. Refolding of gp23 in vitro also requires the GroEL‐gp31 chaperonin complex, which encapsulates the major capsid protein in the Anfinsen cage where it acquires its functional structure. We investigate the kinetics of the folding reaction and show that folding is essentially complete after one minute. We reveal novel dynamic information about the molecular mechanism by which the GroEL‐gp31 chaperonin complex facilitates folding. For the first time, the various stages of the folding reaction have been studied in detail. We find that the folding rate of gp23 inside GroEL‐gp31 complex is faster than that of well‐characterized substrates of GroEL‐GroES. Folding of gp23 requires one round of folding i.e. the reaction is very efficient. We conclude that the unique folding abilities of the GroEL‐gp31 chaperonin complex required during bacteriophage T4 infection are conferred by the physiological substrate gp23 in addition to structural properties of the bacteriophage‐encoded co‐chaperonin, gp31.