What Is Diguanylate Cyclase and How Does It Work?
Learn about diguanylate cyclases, the bacterial enzymes that govern cellular adaptation and are a key focus for novel therapeutic strategies.
Diguanylate cyclases are enzymes found widely across the bacterial domain that synthesize a key signaling molecule. This molecule governs a wide array of cellular activities, acting as a molecular switch in response to environmental and internal cues. By controlling the concentration of this signal, these enzymes enable bacteria to transition between different lifestyles, which is fundamental for survival in fluctuating environments.
Cyclic di-GMP The Key Bacterial Signal
The signaling molecule produced by diguanylate cyclases is bis-(3′-5′)-cyclic dimeric guanosine monophosphate, or cyclic di-GMP (c-di-GMP). This molecule is a near-ubiquitous second messenger in bacteria. Its discovery revealed a complex regulatory network that operates in many bacterial species. The cellular levels of c-di-GMP are carefully managed through its synthesis and degradation to allow for rapid changes.
Diguanylate cyclases (DGCs) synthesize c-di-GMP by catalyzing the condensation of two guanosine triphosphate (GTP) molecules. This reaction occurs in a specific region of the enzyme called the GGDEF domain, named for a conserved amino acid sequence. The process requires the enzyme to form a dimer, which brings two GGDEF domains together to facilitate the reaction.
The activity of these enzymes is not constant and is subject to intricate regulation. Many DGCs are multidomain proteins with sensory domains that detect signals like light, oxygen, or chemicals. These inputs trigger conformational changes that activate or deactivate the catalytic GGDEF domain. The product, c-di-GMP, can also regulate DGC activity through feedback inhibition, binding to an inhibitory site on the enzyme to prevent excessive production.
How Diguanylate Cyclases Direct Bacterial Behavior
The intracellular concentration of c-di-GMP orchestrates major shifts in bacterial lifestyles, particularly the transition between a motile state and a sessile, community-based existence. High levels of c-di-GMP are associated with biofilm formation, which are surface-attached communities encased in a self-produced matrix. The signaling molecule promotes the production of matrix components like polysaccharides and proteins.
Conversely, low levels of c-di-GMP are linked to motility. When c-di-GMP concentrations are high, the machinery required for movement, such as the flagellum, is downregulated. For instance, in Caulobacter crescentus, the DGC known as PleD is involved in turning off flagellar rotation. This inverse relationship allows bacteria to halt movement and colonize a surface.
Beyond the motile-to-sessile switch, c-di-GMP signaling influences other bacterial processes. In many pathogenic bacteria, the expression of virulence factors is controlled by this network. By modulating the production of toxins and adhesion factors, DGCs can impact disease progression. This system allows pathogens to coordinate their attack, activating virulence mechanisms when the bacterial population reaches a sufficient density.
The influence of DGCs also extends to processes like cell cycle progression. In some species, c-di-GMP signaling helps coordinate cell division with environmental conditions. For example, the DGC YfiN in Escherichia coli can act as a cell division inhibitor in response to cellular stress. This protein can relocate to the division site and stall the process, ensuring the cell divides only when conditions are safe.
Diguanylate Cyclases Relevance to Human Disease and Treatment
The control DGCs exert over biofilm formation is relevant to human health. Bacteria in biofilms are difficult to eradicate and cause many persistent and chronic infections. These communities form on medical devices like catheters and implants, and on tissues like the lungs of individuals with cystic fibrosis. The biofilm’s protective matrix shields bacteria from the host immune system and antibiotics.
Because DGCs are instrumental in biofilm formation, they are a focus of research for new antimicrobial therapies. The ability of pathogens like Pseudomonas aeruginosa to cause long-term infections is tied to their capacity to form biofilms. This makes the enzymes that produce c-di-GMP targets for intervention. Disrupting the synthesis of this signaling molecule could prevent bacteria from establishing biofilms.
Therapeutic strategies targeting this signaling pathway offer an alternative to traditional antibiotics. Instead of killing bacteria directly, which can drive resistance, these approaches aim to disarm them. One strategy involves creating small-molecule inhibitors that block the active site of DGCs, preventing c-di-GMP production. This could reduce virulence and disrupt biofilms, making bacteria more vulnerable to antibiotics or the immune system.
Another approach focuses on manipulating c-di-GMP levels to induce bacteria to abandon their protected biofilm lifestyle. Artificially lowering the concentration of this messenger could cause bacteria to disperse from a biofilm, making them susceptible to antimicrobial agents. This type of behavior-modifying drug represents a different way of thinking about antibacterial treatment.