Hydrophilic residues 526KNDAAD531 in the flexible C-terminal region of the chaperonin GroEL are critical for substrate protein folding within the central cavity

The final 23 residues in the C-terminal region of Escherichia coli GroEL are invisible in crystallographic analyses due to high flexibility. To probe the functional role of these residues in the chaperonin mechanism, we generated and characterized C-terminal truncated, double ring, and single ring mutants of GroEL. The ability to assist the refolding of substrate proteins rhodanese and malate dehydrogenase decreased suddenly when 23 amino acids were truncated, indicating that a sudden change in the environment within the central cavity had occurred. From further experiments and analyses of the hydropathy of the C-terminal region, we focused on the hydrophilicity of the sequence region (526)KNDAAD(531) and generated two GroEL mutants where these residues were changed to a neutral hydropathy sequence ((526)GGGAAG(531)) and a hydrophobic sequence ((526)IGIAAI(531)), respectively. Very interestingly, the two mutants were found to be defective in function both in vitro and in vivo. Deterioration of function was not observed in mutants where this region was replaced by a scrambled ((526)NKADDA(531)) or homologous ((526)RQEGGE(531)) sequence, indicating that the hydrophilicity of this sequence was important. These results highlight the importance of the hydrophilic nature of (526)KNDAAD(531) residues in the flexible C-terminal region for proper protein folding within the central cavity of GroEL.