Cold rescue of the thermolabile tailspike intermediate at the junction between productive folding and off‐pathway aggregation

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Abstract
Off‐pathway intermolecular interactions between partially folded polypeptide chains often compete with correct intramolecular interactions, resulting in self‐association of folding intermediates into the inclusion body state. Intermediates for both productive folding and off‐pathway aggregation of the parallel β‐coil tailspike trimer of phage P22 have been identified in vivo and in vitro using native gel electrophoresis in the cold. Aggregation of folding intermediates was suppressed when refolding was initiated and allowed to proceed for a short period at 0 °C prior to warming to 20 °C. Yields of refolded tailspike trimers exceeding 80% were obtained using this temperature‐shift procedure, first described by Xie and Wetlaufer (1996, Protein Sci 5517‐523). We interpret this as due to stabilization of the thermolabile monomeric intermediate at the junction between productive folding and off‐pathway aggregation. Partially folded monomers, a newly identified dimer, and the protrimer folding intermediates were populated in the cold. These species were electrophoretically distinguished from the multimeric intermediates populated on the aggregation pathway. The productive protrimer intermediate is disulfide bonded (Robinson AS, King J, 1997, Nat Struct Biol 4:450‐455), while the multimeric aggregation intermediates are not disulfide bonded. The partially folded dimer appears to be a precursor to the disulfide‐bonded protrimer. The results support a model in which the junctional partially folded monomeric intermediate acquires resistance to aggregation in the cold by folding further to a conformation that is activated for correct recognition and subunit assembly.