Molecule of the Month: Two-component Systems

Bacteria live at the mercy of their environment, much more so than we do. Our bodies help to buffer changes in temperature or resources, and we can always pick up and walk away if things get really bad. Bacteria don't have these options. They are tiny and are subject to large changes in conditions in their local surroundings, so they often must make due with what they have. To do this, they build a collection of several dozen sensors that probe the local conditions and decide how best to respond.

In most cases, this sensory system is quite simple, comprised of two parts. The first component is a large sensor molecule. It includes a domain that senses something-- for instance, the one shown here (PDB entry 3by8 ) senses small nutrients such as citrate. This is connected to a domain that includes a kinase (PDB entry 2c2a ), that can phosphorylate a special histidine near the center of the protein. This phosphoryl group can then be passed to an aspartate on the second component, a "response regulator", which mobilizes the appropriate action in the cell. When the sensor is activated, it sends a signal through the molecule to the kinase domain, which then either increases or decreases the phosphorylation level of the response regulator and controls the adaptive responses.

Looking at the two-component systems in bacterial cells, we find many different variations. All are built with a similar set of parts: an input sensor, an output response regulator, and the phosphorylation machinery that connects the two. But the details are often changed for particular needs. For instance, some are entirely inside the cell, and others have a sensor outside. Some have a separate response regulator that delivers the message throughout the cell, others have it built into the sensor chain. The mechanism for communicating between the sensor and the kinase is a great mystery, under intensive study. Some scientists propose a scissoring motion that propagates down the molecule, others propose a piston-like motion.

Once the signal is received, a response regulator delivers the message inside the cell. In many cases, the response regulator is a DNA-binding protein. When the phosphate is added, the protein enhances its binding to DNA and modifies the transcription of appropriate genes for the response. For instance, PDB entry 1kgs shows the inactive, unphosphorylated form of a response regulator and PDB entries 1gxp and 1zes show the active form of a similar regulator bound to DNA. Other regulators, however, act quite differently, binding to RNA, acting as enzymes, or binding to other proteins--whatever is needed to stimulate the best response.