Search

Quorum sensing in Vibrio cholerae involves signaling between two-component sensor protein kinases and the response regulator LuxO to control the expression of the master regulator HapR. HapR, in turn, plays a central role in regulating a number of important processes, such as virulence gene expression and biofilm formation. We have determined the crystal structure of HapR to 2.2-[Angstrom] resolution. Its structure reveals a dimeric, two-domain molecule with an all-helical structure that is strongly conserved with members of the TetR family of transcriptional regulators. The N-terminal DNA-binding domain contains a helix-turn-helix DNA-binding motif and alteration of certain residues in this domain completely abolishes the ability of HapR to bind to DNA, alleviating repression of both virulence gene expression and biofilm formation. The C-terminal dimerization domain contains a unique solvent accessible tunnel connected to an amphipathic cavity, which by analogy with other TetR regulators, may serve as a binding pocket for an as-yet-unidentified ligand.

The mechanism by which motor proteins hydrolyze ATP and move along cytoskeletal filaments is still unknown. One approach to deciphering the mechanism is to correlate steps of ATP hydrolysis with structural states of the motors to determine the changes the motors undergo during the hydrolysis cycle. Unfortunately, available crystal structures represent only a few steps of the cycle and obtaining atomic structures that represent the motors bound to their filament has been difficult. Now, two new myosin crystal structures have been reported that show features expected for myosin motors bound in rigor to actin. The two new structures show changes at both the actin-binding surface and the active site that have not been observed previously.

Research on microtubule-based motor proteins, using X-ray crystallography, reveals that the human kinesin motor domain and the core of the catalytic domain of the actin-based motor myosin exhibit structural similarities. These two classes of mechanochemical enzymes have a common origin and perform similar force-generating tasks, such as organelle transport and chromosome segregation. A single alpha/beta arrow head-shaped domain with a dimension of 70 x 45 x 45 angstrom composes most of the kinesin motor domain's structure.

The crystal structure of microtubule-based ATPases of the kinesin superfamily reveals that the ncd and kinesin motor domains have similar structures. X-ray crystallography is used to determine the ncd structure in Drosophila at 2.5 angstrom resolution. These motors share a common microtubule-binding site as is evident from the locations of conserved surface amino acids. A similar method of changing between NTP and NDP state conformations for NTPases is indicated by a comparison of ncd, kinesin, myosin and G proteins.

HapR is a TetR‐family transcriptional regulator that controls quorum sensing in Vibrio cholerae, the causative agent of cholera. HapR regulates the expression of hemagglutinin protease, virulence and biofilm genes. The crystal structure of wild‐type HapR from V. cholerae strain O1 El Tor C6706 has previously been solved. In this study, the structure of a DNA‐binding‐deficient variant of HapR (HapRV2) derived from the protease‐deficient V. cholerae serotype O37 strain V2 is reported. The structure reveals no structural differences compared with wild‐type HapR. However, structural alignment of HapRV2 with the TetR‐family member QacR in complex with its operator DNA suggests that the aspartate residue located between the regulatory and DNA‐binding domains may clash with and electrostatically repel the phosphate backbone of DNA to prevent binding. [ABSTRACT FROM AUTHOR]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.cell.2008.09.005 Byline: Jared C. Cochran (1), F. Jon Kull (1) Abstract: The processive movement of the dimeric motor protein kinesin 1 along microtubules requires communication between the two motor domains. now show that tension between the motor domains not only is necessary for normal processivity but also may be sufficient for motor motility under some conditions. Author Affiliation: (1) Department of Chemistry, Dartmouth College, Hanover, NH 03755, USA

ToxR is a transmembrane transcription factor that is essential for virulence gene expression and human colonization by Vibrio cholerae. ToxR requires its operon partner ToxS, a periplasmic integral membrane protein, for full activity. These two proteins are thought to interact through their respective periplasmic domains, ToxRp and ToxSp. In addition, ToxR is thought to be responsive to various environmental cues, such as bile salts and alkaline pH, but how these factors influence ToxR is not yet understood. Using NMR and reciprocal pull down assays, we present the first direct evidence that ToxR and ToxS physically interact. Furthermore, using NMR and DSF, it was shown that the bile salts cholate and chenodeoxycholate interact with purified ToxRp and destabilize it. Surprisingly, bile salt destabilization of ToxRp enhanced the interaction between ToxRp and ToxSp. In contrast, alkaline pH, which is one of the factors that leads to ToxR proteolysis, decreased the interaction between ToxRp and ToxSp. Taken together, these data suggest a model whereby bile salts or other detergents destabilize ToxR, increasing its interaction with ToxS to promote full ToxR activity. Subsequently, as V. cholerae alkalinizes its environment in late stationary phase, the interaction between the two proteins decreases, allowing ToxR proteolysis to proceed. [ABSTRACT FROM AUTHOR]