SELECTIVE IN-VIVO RESCUE BY GROEL ES OF THERMOLABILE FOLDING INTERMEDIATES TO PHAGE-P22 STRUCTURAL PROTEINS/

The in vivo conformational substrates of the GroE chaperonins have bee n difficult to identify, in part because of limited information on in vivo polypeptide chain folding pathways. Temperature-sensitive folding (tsf) mutants have been characterized for the coat protein and tailsp ike protein of phage P22. These mutations block intracellular folding at restrictive temperature by increasing the lability of folding inter mediates without impairing the stability or function of the native sta te. Overexpression of GroEL/ES suppressed the defects of tsf mutants a t 17 sites in the coat protein, by improving folding efficiency rather than assembly efficiency or protein stability. Immunoprecipitation ex periments demonstrated that GroEL interacted transiently with newly sy nthesized mild-type coat protein and that this interaction was prolong ed by the tsf mutations. Folding defects of the tailspike polypeptide chains were not suppressed. A fraction of the tsf mutant tailspike cha ins bound to GroEL but were inefficiently discharged. The results sugg est that 1) thermolabile folding intermediates are natural substrates of GroEL/ES; 2) although GroEL may bind such intermediates for many pr oteins, the chaperoning function is limited to a subset of substrate p roteins; and 3) a key reason for the heat-shock response may be to sta bilize thermolabile folding intermediates at elevated temperatures.