How do bacteria make decisions? Part 4: Getting the message

chemical structure of cyclic diguanylate (Cycl...
chemical structure of cyclic diguanylate (Cyclic di-GMP; c-di-GMP; 3′,5′-Cyclic diguanylic acid; 5GP-5GP) (Photo credit: Wikipedia)

Welcome to part 4 of how bacteria make decisions. Parts 1 and 2 dealt with chemotaxis. Part 3 was a look at two component signaling systems. This part will deal with my favorite aspect of bacterial decisions for several reasons. Second messengers are common from bacteria to humans. The major second messenger we all learn about in biochemistry class is cyclic AMP (cAMP). However, bacteria use several nucleotides as second messengers. Many are used as determinants in the decision making process, but one of the most recently discovered (and personal favorite) is cyclic-di-GMP, or c-di-GMP.

Bacteria are constantly processing signals both inside and outside their cell membranes. Hard to believe that one of the most abundant response molecules was only discovered in the late 1980s while researching how a certain species, Acetobacter xylinum now known as Gluconacetobacter xylinus, produced cellulose. Almost by accident, the Benziman lab discovered the enzyme responsible for cellulose production (cellulose synthase) was regulated by a nucleotide, later found to be c-di-GMP. Since that discovery, c-di-GMP has become a hot topic among microbiologists and immunologists due to the decisions bacteria make as the level of c-di-GMP increases within the cell. As I learned it, the concentration of c-di-GMP had predictable outcomes on the decisions of bacteria: high levels leads to loss of motility, increase in biofilm formation, changes in cell morphology, and increase in cell-cell communication. When low levels of c-di-GMP are present, the cell decides to move around (motility), become resistant to heavy metals, and, most importantly, becomes virulent. For example, Vibrio cholerae, the bad guy responsible for cholera, only decides to move around and produce cholera toxin when c-di-GMP levels are low in the cell. If levels increase, V. cholerae will produce biofilms via extracellular polysaccharide (EPS) production.

You might be asking yourself what controls the c-di-GMP levels of a bacterial cell. The initial discovery in the Benziman lab also found the enzymes/proteins that were responsible for making and breaking the second messenger. The long (and short) names are; for making c-di-GMP from 2 GTP molecules, diguanylate cyclases (DGCs aka GGDEF proteins) and degradation by phosphodiesterases (PDEs aka EAL proteins). GGDEF and EAL proteins are so called due to important amino acids necessary for their functions, GGDEF is glycine, glycine, aspartate, glutamate, phenylalanine, and EAL is glutamate, alanine, leucine. These enzymatic activities are usually controlled by regulatory protein domains common in bacteria (and humans). Signals from the environment (internal or external) can trigger changes in enzyme activity of GGDEFs and EALs thus changing the cellular concentration of c-di-GMP. This mechanism is well understood after 30 years of research. However, what happens next is still essentially unknown.

Cyclic-di-GMP levels rise within a bacterial cell. Now what? It was known early on that c-di-GMP itself could then interact with GGDEFs to inhibit activity. But what other proteins interact with c-di-GMP and help these bacteria decide to make major lifestyle changes? It wasn’t until 2006 that bioinformaticians predicted c-di-GMP binding to a protein, or protein domain. PilZ, an obscure protein of unknown function but necessary for Type IV pili motility, was hypothesized to bind c-di-GMP. By the end of 2007, this prediction was verified and PilZ domain proteins were the first shown linking c-di-GMP to downstream proteins in pathways, or circus rings.

Transitioning from a free swimming/moving cell to life in a biofilm community is a major lifestyle change for bacteria. This decision takes commitment which is initiated by a small molecule. In the next installment, we will get to the heart of current research.

English: Crystal structure of diguanylate cycl...
English: Crystal structure of diguanylate cyclase PleD in complex with c-di-GMP from Caulobacter crecentus (Photo credit: Wikipedia)
bacterial second messenger, cyclic-di-GMP, bacteria, microbiology
My interpretation of c-di-GMP as created by Divvr web app.

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