Maria Silow, Avd. för Biokemi Ideon G2:G Sölvegatan 41 223 63 Lund

Mia Silow and Mikael Oliveberg, Biochemistry, Chemical Centre,
BOX 124, S-221 00 Lund, Sweden.

Recently some small globular proteins have been found
to fold directly into their native conformations in a two-state
process without populated intermediates. The findings question the
role of intermediates and point at the possibility that partly
structured states are misfolds trapped in non-productive pathways. We
report here that the spliceosomal protein U1A, which exhibits a
classical three-state folding behaviour, has an intermediate which is
caused by transient aggregation of denatured protein. The aggregation
behaviour is reflected in a concentration dependence of the refolding
amplitudes. At low concentrations of protein, U1A folds in a
two-state process directly from the denatured monomer whereas at high
concentrations of protein the observed refolding takes place from an
aggregate which is formed in the dead-time of the stopped-flow
instrument. Is transient aggregation a general problem in time
resolved folding studies or a unique feature of the polypeptide of U!
1A ?

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