Crystal Structure of the Resuscitation-Promoting Factor D DUFRpfB from M. tuberculosis

Mycobacterium tuberculosis is able to establish a non-replicating state and
survive in an intracellular habitat for years. Resuscitation of dormant M.
tuberculosis bacteria is promoted by resuscitation-promoting factors (Rpfs),
which are secreted from slowly replicating bacteria close to dormant bacteria.
Here we report the crystal structure of a truncated form of RpfB
(residues 194-362), the sole indispensable Rpf of the five Rpfs encoded in
this bacterium genome. The structure, denoted as ?DUFRpfB, exhibits a
comma-like shape formed by a lysozyme-like globular catalytic domain and
an elongated G5 domain, which is widespread among cell surface binding
proteins. The G5 domain, whose structure was previously uncharacterised,
presents some peculiar features. The basic structural motif of this domain,
which represents the tail of the comma-like structure, is a novel supersecondary-
structure element, made of two ?-sheets interconnected by a
pseudo-triple helix. This intricate organisation leads to the exposure of
several backbone hydrogen-bond donors/acceptors. Mutagenesis analyses
and solution studies indicate that this protein construct as well as the fulllength
form are elongated monomeric proteins. Although ?DUFRpfB does
not self-associate, the exposure of structural elements (backbone H-bond
donors/acceptors and hydrophobic side chains) that are usually buried in
globular proteins is typically associated with adhesive properties. This
suggests that the RpfB G5 domain has a cell-wall adhesive function, which
allows the catalytic domain to be properly oriented for the cleavage
reaction. Interestingly, sequence comparisons indicate that these structural
features are also shared by G5 domains involved in biofilm formation.