pH jump studies of the folding of the multidomain ribosomal protein L9: The structural organization of the N-terminal domain does not affect the anomalously slow folding of the C-terminal domain

The folding kinetics of the multidomain ribosomal protein L9 were studied using pH jump stopped-flow fluorescence and circular dichroism (CD) in conjunction with guanidine hydrochloride (GdnHC1) jump stopped-flow CD experiments. Equilibrium CD and 1D ¹H NMR measurements demonstrated that the C-terminal domain unfolds below pH 4 while the N-terminal domain remains fully folded. Thus, the N-terminal domain remains folded during the pH jump experiments. The folding rate constant of the C-terminal domain was determined to be 3.5 s^-1 by pH jump experiments conducted in the absence of denaturant using stopped-flow CD and fluorescence. CD-detected GdnHC1 jump measurements showed that the N- and C-terminal domains fold independently each by an apparent two-state mechanism. The folding rate constant for the N- terminal domain and the C-terminal domain in the absence of denaturant were calculated to be 760 and 4.7 s^-1, respectively. The good agreement between the pH jump and the denaturant concentration jump experiments shows that the folding rate of the C-terminal domain is the same whether or not the N- terminal domain is folded. This result suggests that the slow folding of the C-terminal domain is not a consequence of unfavorable interactions with the rest of the protein chain during refolding. This is an interesting result since contact order analysis predicts that the folding rate of the C-terminal domain should be noticeably faster. The folding rate of the isolated N- terminal domain was also measured by stopped-flow CD and was found to be the same as the rate for the domain in the intact protein.