Inherent chaperone-like activity of aspartic proteases reveals a distant evolutionary relation to double-c barrel domains of AAA-ATPases

Chaperones and proteases share the ability to interact with unfolded proteins. Here we show that enzymatically inactive forms of the aspartic proteases HIV-1 protease and pepsin have inherent chaperone-like activity and can prevent the aggregation of denatured substrate proteins. In contrast to proteolysis, which requires dimeric enzymes, chaperone- like activity could be observed also with monomeric domains. The involvement of the active site cleft in the chaperone- like function was demonstrated by the inhibitory effect of peptide substrate inhibitors. The high structural similarity between aspartic proteases and the N-terminal double-psi barrels of Cdc48-like proteins, which are involved in the unfolding and dissociation of proteins, suggests that they share a common ancestor. The latent chaperone- like activity in aspartic proteases can be seen as a relic that has further evolved to serve substrate binding in the context of proteolytic activity.