Why ask this?
Almost every bacterium divides using FtsZ, a protein that assembles into a ring (the Z-ring) at the middle of the cell. FtsZ is a distant cousin of tubulin, the protein that builds our own cells’ skeleton, and it only burns its fuel, GTP, once it has assembled into filaments. Because the Z-ring is essential for bacteria and differs from anything in our cytoplasm, it is a long-standing target for new antibiotics. Established: Replicated many times; textbook knowledge.
Measuring how fast FtsZ uses GTP tells me how its filaments behave. The classic trick is to detect the phosphate released, one molecule per GTP used.
FtsZ, the bacterial division protein
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FtsZ from Spiroplasma melliferum with GDP, PDB 7YSZ (Chakraborty et al. 2024, J Biol Chem). Model: khushisingh.in, CC BY 4.0.
The tool: a protein that glows for phosphate
The E. coli phosphate-binding protein snaps shut around phosphate like a Venus flytrap. Swap one amino acid for a cysteine, attach the dye MDCC, and the dye brightens when the trap closes. This turns an invisible molecule into light you can measure every few seconds (Brune et al. 1994). Established: Replicated many times; textbook knowledge.
Phosphate-binding protein: open and closed
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PDB 1A54: the sensor with its MDCC dye and a phosphate bound.
Phosphate-binding protein A197C labelled with MDCC, bound to phosphate, PDB 1A54 (Hirshberg et al. 1998, Biochemistry). Model: khushisingh.in, CC BY 4.0. Phosphate-binding protein in the open, phosphate-free form, PDB 1OIB. Superimposed on 1A54. Model: khushisingh.in, CC BY 4.0.
- 1 · A protein that changes shape E. coli phosphate-binding protein: two lobes that close on phosphate
- 2 · Add one cysteine Alanine 197 becomes cysteine (A197C), at the edge of the cleft
- 3 · Attach a dye The dye MDCC is linked to that cysteine; open, it glows weakly
- 4 · Phosphate closes it The lobes clamp shut and the dye glows about 5 times brighter
From light to kinetics
Each GTP that FtsZ splits releases one phosphate, so the sensor’s brightness rises as FtsZ works. A calibration with known amounts of phosphate turns brightness into concentration, and the slope over time gives the rate. Because FtsZ hydrolyses GTP mainly once it has assembled into filaments, almost nothing happens below a critical concentration of FtsZ (Romberg & Levin 2003). Established: Replicated many times; textbook knowledge. Above it, rates measured at several GTP concentrations give the Michaelis–Menten parameters Vmax and Km.
From light to kinetics. With a calibration from known phosphate amounts, the slope of each fluorescence trace becomes a rate of GTP use. Rates at several GTP concentrations fit the Michaelis–Menten curve, giving Vmax and Km. Schematic traces, not data.
What I did
- I prepared the sensor: mutagenesis, expression, purification, labelling and quality checks.
- I calibrated how its fluorescence changes with phosphate.
- I used it to follow GTP use by FtsZ from Spiroplasma melliferum (a cell-wall-less bacterium studied in the lab) under different conditions, and wrote a step-by-step analysis pipeline from raw fluorescence to Michaelis–Menten parameters.
What I took away
- Good measurement is a design problem: contamination (phosphate is everywhere!), calibration ranges and blanks decide what you can trust.
- Real enzymes are richer than textbooks. Because FtsZ only uses GTP once it polymerises, it does not obey the simplest enzyme model, and that is exactly what makes it interesting.
Why it matters
New antibiotics need new targets. FtsZ inhibitors have reached early clinical testing, and none has yet passed Phase 2. Sensitive, real-time assays like this one are how candidate molecules get screened.