Structure and Function of Two Putative Virulence Factors from Francisella Tularensis

F. tularensis is a poorly understood category A priority pathogen and the etiological agent of Tularemia, or rabbit fever. Previous work by coworkers at LLNL identified seven putative virulence factor proteins, for which most have weak homology to proteins of known function. These seven proteins presumably are secreted host effector proteins, as they are important for inducing an encysted morphology in F. tularensis infected amoeba. Subsequent genetic studies have further implicated some of these ORFs as having a role in infection of human cells in culture. We have applied a structural-functional genomics approach in an attempt to elucidate the function of these rapid encystment proteins (REPs). Here we present the crystal structures of REP24 (24 kD) and REP34 (34 kD) and report on their putative function. REP24 appears to be a cysteine protease with a novel arrangement of the active-site catalytic triad, and we show that it binds to the cysteine protease inhibitor, JPM-565, both in vitro and in crystallo. REP34 By structural homology and, REP34 appears to be a Zn-dependent carboxypeptidase that possesses a novel fold that deviates from related funnelins, resulting in a rearranged active and substrate-recognition sites. We show the definitive presence of a Zn by anomalous diffraction, and catalytic activity of the enzyme using hippuryl-L-Arg. We believe that these structures provide potential starting points for rational design of new therapeutics against tularemia and, combined with further biochemical evidence, will provide important insights into the molecular mechanisms of F. tularensis infection.