The function of propeptide domains of cysteine proteinases.

The papain-like cysteine proteinases can be divided into cathepsin L-like and cathepsin B-like enzymes because of the extended proregion of the former ones. We performed a series of mutations (alanine scan) in the prodomain of procathepsin S in order to elucidate the function of this extended domain in the L-like cathepsins. One of the most striking results was that the structural stability and the folding of procathepsin S were considerably dependent on an aromatic stack built by the residues Trp 28, Trp 31 and Trp 52. Replacement either of one or of all of these residues by alanine resulted in loss of transport, maturation and secretion of the mutated zymogen. Recombinant propeptides carrying the same mutations are not longer selective and powerful inhibitors of cathepsin S. Therefore we postulated an essential role of the propeptide for proper folding of the whole enzyme. This assumption was further proved by in vitro studies. We investigated the capability of recombinant cathepsin S propeptide to catalyze the renaturation of denatured mature cathepsin S. The experiments showed a 10-25 fold faster renaturation rate in presence than in absence of the propeptide underlining its function as intramolecular chaperone.