Functional products of gene expression are generally obtained only after the initial linear polypeptide chain has folded to its native three-dimensional conformation (Creighton 1984). Unfolded proteins are usually inac-tive biologically. Folding is believed to be a sponta-neous self-assembly process, directed solely by the amino acid sequence of the polypeptide chain under the appropriate physiological conditions, and occurs short-ly after assembly of the polypeptide chain on the ribosome. Most studies of protein folding have been carried out in vitro, using the intact polypeptide chain. Even under such well-controlled conditions, elucidating the nature of the folding transition has been hampered by its cooperativity: Partially folded intermediates are un-
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Creighton et al. (1987) studied this question.