Intrinsically disordered proteins in nickel trafficking

Nickel is an essential component in the active site of several enzymes. On the other hand, the toxicity of this metal for cellular life is well demonstrated. Nickel-dependent organisms, including some deadly bacteria, have developed sophisticated mechanisms and efficient intracellular carriers for delivering nickel into the right sub-cellular location, avoiding its extremely high toxicity and overcoming its scarcity in natural environments. The preclusion of regular intracellular nickel trafficking can be envisaged as a strategy against nickel-dependent pathogens. Among nickel-containing enzymes, urease is an ideal anti-bacterial target, since it is one of the most important virulence factors for ureolytic bacteria, as well as the final destination of the greater part of their intracellular nickel. The in vivo urease activation requires four conserved helper proteins, named UreD, UreE, UreF and UreG, which ensure the correct sequence of events needed to assemble the catalytic nickel-site. Bacillus pasteurii UreE, whose crystallographic structure has been determined (1), is responsible for nickel delivery into the apo-urease precursor through a metal site able to bind two nickel ions (2). BpUreG is a GTPase, essential for the assembly of urease active site. BpUreG is a dimer in solution, containing some degree of secondary and tertiary structure, as revealed by light-scattering, CD and fluorescence spectroscopy (3,4). However, NMR spectroscopy indicates that native BpUreG does not possess a rigid tertiary structure, existing in fast equilibrium between different conformations, with large portions of unfolded backbone. Therefore, this protein has been ascribed to the class of intrinsically disordered proteins. Analogously, this partially unstructured behavior has been observed for Mycobacterium tuberculosis UreG, suggesting that this is a general feature for all UreG chaperones (5). Recently, structural and functional studies on Helicobacter pylori UreG showed some distinctive features of this protein. The properties observed in UreG proteins from different organisms evidenced the important role that protein disorder plays in the regulatory mechanisms responsible for correct intra-cellular nickel trafficking. The discovery of a possible function for UreF as a GAP (GTP-ase Activating Protein), possibly promoting the folding and the activation of UreG, is consistent with this view (6)