Structural characterization and modeling of the Borrelia burgdorferi hybrid histidine kinase Hk1 periplasmic sensor: A system for sensing small molecules associated with tick feeding

Abstract

Two-component signal transduction systems are the primary mechanisms by which bacteria perceive and respond to changes in their environment. The Hk1/Rrp1 two-component system (TCS) in B. burgdorferi consists of a hybrid histidine kinase and a response regulator with diguanlyate cyclase activity, respectively. Phosphorylated Rrp1 catalyzes the synthesis of c-di-GMP, a second messenger associated with bacterial life-style control networks. Spirochetes lacking either Hk1 or Rrp1 are virulent in mice but destroyed within feeding ticks. Activation of Hk1 by exogenous stimuli represents the seminal event for c-di-GMP signaling. We reasoned that structural characterization of Hk1's sensor would provide insights into the mechanism underlying signal transduction and aid in the identification of activating ligands. The Hk1 sensor is composed of three ligand-binding domains (D1-3), each with homology to periplasmic solute-binding proteins (PBPs) typically associated with ABC transporters. Herein, we determined the structure for D1, the most N-terminal PBP domain. As expected, D1 displays a bilobed Venus Fly Trap-fold. Similar to the prototypical sensor PBPs HK29S from Geobacter sulfurreducens and VFT2 from Bordetella pertussis, apo-D1 adopts a closed conformation. Using complementary approaches, including SAXS, we established that D1 forms a dimer in solution. The D1 structure enabled us to model the D2 and D3 domains. Differences in the ligand-binding pockets suggest that each PBP recognizes a different ligand. The ability of Hk1 to recognize multiple stimuli provides spirochetes with a means of distinguishing between the acquisition and transmission blood meals and generate a graded output response that is reflective of the perceived environmental threats.

Cyclic di-GMP Modulates Gene Expression in Lyme Disease Spirochetes at the Tick-Mammal Interface To Promote Spirochete Survival during the Blood Meal and Tick-to-Mammal Transmission

“…Ligand binding by sensor PBPs is thought to induce a conformational change (e.g., from open to closed) within the histidine kinase sensor which, when transmitted across the IM, activates the histidine kinase core (132)(133)(134). Nonsynonymous amino acid differences between the putative ligand binding pockets for Hk1's three sensor PBP domains suggest that each binds a different ligand (131). Thus, we envision at least two possible explanations for the increased expression of c-di-GMP-regulated genes in larvae versus nymphs.…”

A novel bacterial l‐arginine sensor controlling c‐di‐GMP levels in Pseudomonas aeruginosa

“…In general, PBPs trap nutrients for their internalization into bacteria by ABC transporters; they are commonly in the same operon of the permease counterpart . Nevertheless, the PBP‐fold and specificity is conserved also when fused to diverse sensory/transduction systems, not necessarily linked to the import event (ie, the so‐called “sensor‐PBPs”), conserved both in prokaryotes and in eukaryotes …”

Interaction of the Lyme disease spirochete with its tick vector

“…The Hk1 periplasmic sensor consists of three ligand-binding domains, each with homology to ABC transporter periplasmic solute-binding proteins (Bauer et al, 2015). Analyses of the Hk1 sensor point to amino acids and/or their derivatives as potential activating ligands (Bauer et al, 2015). Upon ligand binding, Hk1 is thought to mediate a signal transduction cascade that culminates in phosphorylation of Rrp1.…”