Induced Folding Under Membrane Mimetic and Acidic Conditions Implies Undiscovered Biological Roles of Prokaryotic Ubiquitin-Like Protein Pup

Ubiquitin-like proteins play important roles in diverse biological processes. In Mycobacterium tuberculosis, Pup (prokaryotic ubiquitin-like protein), a functional homologue of eukaryotic ubiquitin, interacts with the proteasome ATPase subunit Mpa to recognize and unfold substrates, and then translocate them into the proteasome core for degradation. Previous studies revealed that, Pup, an intrinsically disordered protein (IDP), adopts a helical structure upon binding to the N-terminal coiled-coil domain of Mpa, at its disordered C-terminal region. In the present study, using circular dichroism (CD), surface plasmon resonance (SPR) and nuclear magnetic resonance (NMR), we show that membrane mimetic and acidic conditions also induce Pup to adopt helical conformations. Moreover, at low pH, Pup, via both of its N- and C-terminal regions, binds to Mpa on sites from the N-terminal region in addition to the C-terminal region of the coiled-coil domain. Our results imply Pup may play undiscovered roles in some biological processes e.g. those involve in membrane.