No evidence for a forced-unfolding mechanism during ATP/GroES binding to substrate-bound GroEL: no observable protection of metastable Rubisco intermediate or GroEL-bound Rubisco from tritium exchange

In tritium-hydrogen exchange experiments, the large GroEL substrate Rubisco was unfolded and exchanged in urea/ acid/tritiated water, then diluted into either protic buffer or protic buffer containing GroEL. The respective Rubisco metastable folding intermediate or Rubisco-GroEL binary complex was then separated from residual tritium after varying times of exchange by centrifugation through P-10 or G-25 resin. No significant tritium was recovered in either case, in contrast to an earlier report. Thus, although the earlier-proposed forced unfolding mechanism for the action of GroEL on a bound polypeptide, occurring during ATP/GroES binding, remains an attractive hypothesis, the data here do not provide any indication that it is involved in the folding of Rubisco. (C) 2005 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.