We describe a basic protocol for quantitating in vitro ATPase activity. This protocol can be optimized based on the level of activity and requirements for a given purified ATPase.
Adenosine triphosphate-hydrolyzing enzymes, or ATPases, play a critical role in a diverse array of cellular functions. These dynamic proteins can generate energy for mechanical work, such as protein trafficking and degradation, solute transport, and cellular movements. The protocol described here is a basic assay for measuring the in vitro activity of purified ATPases for functional characterization. Proteins hydrolyze ATP in a reaction that results in inorganic phosphate release, and the amount of phosphate liberated is then quantitated using a colorimetric assay. This highly adaptable protocol can be adjusted to measure ATPase activity in kinetic or endpoint assays. A representative protocol is provided here based on the activity and requirements of EpsE, the AAA+ ATPase involved in Type II Secretion in the bacterium Vibrio cholerae. The amount of purified protein needed to measure activity, length of the assay and the timing and number of sampling intervals, buffer and salt composition, temperature, co-factors, stimulants (if any), etc. may vary from those described here, and thus some optimization may be necessary. This protocol provides a basic framework for characterizing ATPases and can be performed quickly and easily adjusted as necessary.
ATPases are integral enzymes in many processes across all kingdoms of life. ATPases act as molecular motors that use the energy of ATP hydrolysis to power such diverse reactions as protein trafficking, unfolding, and assembly; replication and transcription; cellular metabolism; muscle movement; cell motility; and ion pumping1-3. Some ATPases are transmembrane proteins involved in transporting solutes across membranes, others are cytoplasmic and may be associated with a biological membrane such as the plasma membrane or those of organelles.
AAA+ ATPases (ATPases associated with various cellular activities) make up a large group of ATPases that share some sequence and structural conservation. These proteins contain conserved nucleotide binding motifs such as Walker-A and -B boxes and form oligomers (generally hexamers) in their active state1. Large conformational changes in these proteins upon nucleotide binding have been characterized among diverse members of the AAA+ family. EpsE is a AAA+ ATPase and member of the bacterial Type II/IV secretion subfamily of NTPases4-6. EpsE powers Type II Secretion (T2S) in Vibrio cholerae, the causative agent of cholera. The T2S system is responsible for the secretion of a wide variety of proteins, such as the virulence factor cholera toxin that causes profuse watery diarrhea when V. cholerae colonizes the human small intestine7.
Techniques for quantitating in vitro ATPase activity are varied, but commonly measure phosphate release using colorimetric, fluorescent, or radioactive substrates8-11. We describe a basic method for determining in vitro ATPase activity of purified proteins using a colorimetric assay based on a commercially available malachite green-containing substrate that measures liberated inorganic phosphate (Pi). At low pH, malachite green molybdate forms a complex with Pi and the level of complex formation can be measured at 650 nm. This simple and sensitive assay may be used to functionally characterize new ATPases and to evaluate the roles of potential activators or inhibitors, to determine the importance of domains and/or specific residues, or to assess the effect of particular treatments on enzymatic activity.
1. Perform ATP Hydrolysis Reaction with Purified Protein
2. Incubate Samples Containing Free Pi with Detection Reagent
3. Quantitate Results Using a Microplate Reader
The in vitro activity of the T2S ATPase EpsE can be stimulated by copurification of EpsE with the cytoplasmic domain of EpsL (EpsE-cytoEpsL) and addition of the acidic phospholipid cardiolipin12. It is also possible to determine the role of particular EpsE residues in ATP hydrolysis by comparing activity of wild type (WT) to variant forms of the protein using this assay. Here, the effect of substituting two lysine residues in the EpsE zinc-binding domain is measured by comparing ATPase activity of purified WT EpsE-cytoEpsL to the EpsE K417AK419A-cytoEpsL variant. In Figure 1, phosphate release proceeds linearly over the course of a 1 hr kinetic assay, a requirement for accurate quantification of kinetic ATPase activity. Figure 2 shows data from the same purified protein samples as Figure 1 that were assayed three times in duplicate and quantitated in terms of the mean rate of phosphate release. These data show that K417 and K419 appear to be important for EpsE ATPase activity. However, comparison of unstimulated (no cardiolipin) ATPase activity rates (Figure 3) shows that K417 and K419 do not contribute to EpsE's basal activity but rather to the ability of the protein to be stimulated by cardiolipin. The positively charged lysines in the EpsE zinc-binding domain may directly interact with the negatively charged phospholipids, thus contributing to the phospholipid-mediated stimulation of EpsE ATPase activity.
Figure 1: Phosphate is Released Linearly in a Kinetic ATPase Assay. One hour kinetic cardiolipin-stimulated ATPase assay comparing phosphate release of 0.5 µM WT EpsE-cytoEpsL to EpsE K417AK419A-cytoEpsL with bovine serum albumin (BSA) assayed as a negative control. Data [amount of released phosphate (y-axis) versus time (x-axis)] were plotted and subjected to linear regression analysis. The slope represents the rate of ATP hydrolysis. A representative graph with linear regression equations for the three proteins is shown. Please click here to view a larger version of this figure.
Figure 2: Double Lysine Mutations in EpsE Zinc-Binding Domain Reduce Stimulated ATPase Activity. Results of the 1 hr kinetic cardiolipin-stimulated ATPase assay with the same proteins and conditions as in Figure 1. Assays were performed three separate times in technical duplicate and the rate of ATP hydrolysis was calculated as nmol phosphate generated per minute per µM protein using linear regression equations. The mean results with standard error are displayed. Please click here to view a larger version of this figure.
Figure 3: Double Lysine Mutations in EpsE Zinc-Binding Domain Do Not Interfere with Unstimulated ATPase Activity. Overnight (16 hr) endpoint unstimulated (no cardiolipin) ATPase assay comparing the activity of 5 µM WT EpsE-cytoEpsL to EpsE K417AK419A-cytoEpsL with BSA as a negative control. Assays were performed three separate times in technical duplicate and mean results with standard error is shown. The basal level of unstimulated EpsE ATPase activity assayed over a 16 hr period is ~1,000-fold lower than 1 hr kinetic cardiolipin-stimulated activity. Please click here to view a larger version of this figure.
This is a general protocol for measuring in vitro ATPase activity of purified proteins for biochemical characterization. This method is easily optimized; for example, adjusting the amount of protein, buffer and salt compositions, temperature, and varying the assay length and intervals (including increasing the total number of intervals) can improve activity quantitation. Commercially available malachite green-based reagents are highly sensitive, and can detect small amounts of free phosphate (~50 pmol in 100 µl). Because of this assay's sensitivity, it is crucial to use disposable plastic ware, ultrapure water, buffers, and reagents devoid of contaminating phosphate. After purifying proteins, size exclusion or ion exchange chromatography is recommended to improve protein purity and remove contaminants.
For proteins displaying weak in vitro ATPase activity, stimulants may be added to the reaction to enhance enzymatic activity. Many factors that stimulate ATPase activity have been characterized. For example, cardiolipin and other membrane lipids, client proteins of chaperones such as Hsp90, and other proteins involved in supporting particular conformations or environments in which ATPases function13-15. Our laboratory first characterized EpsE by purifying monomers with weak ATPase activity compared to homologous ATPases6. We later discovered that when EpsE was copurified with the cytoplasmic domain of EpsL, a transmembrane protein and binding partner of EpsE in the T2S system, addition of acidic phospholipids such as cardiolipin to the reaction mixture greatly increased the ATPase activity of EpsE12. This likely mimics the conditions EpsE experiences at the cytoplasmic membrane that may promote oligomerization.
Many techniques have been used to quantify in vitro ATPase activity of purified proteins. Radioactive γ-32P has been frequently and successfully implemented to quantify ATP hydrolysis14,16, however, safety is a concern and requires approval for the laboratory use of radioactivity. While highly sensitive, the short half-life of γ-32P is also a disadvantage. Other commercial phosphate detection methods are available, such as those that rely on the formation of a fluorescent product. Some of these methods are also very sensitive, but frequently require the addition of other enzymes to the reaction, resulting in reagents that are less stable over time. Additionally, kits are available that detect ADP released during ATP hydrolysis using a stable luminescence-based reagent, but these may be less ideal for ATPases with low levels of activity17.
The assay described here consists of only one phosphate release measurement step, is highly sensitive, and can typically be performed within a few hr. It is also possible to prepare a malachite green-containing substrate to avoid purchasing a kit from a commercial vendor6,9. One consideration before undertaking this assay is that the step between adding the phosphate detection reagent and taking absorbance readings is relatively time-sensitive and must be between 20-30 min. This basic protocol can be used to determine the role of stimulants (as described), antagonists, subunits, domains, and specific residues in ATPase activity15,16,18,19. This assay can also be extended to measure the activity of phosphatases or other enzymes that release phosphate during catalysis. Additionally, this method can be applied to high-throughput screening for ATPase inhibitors20.
The authors have nothing to disclose.
The authors would like to acknowledge funding from a National Institutes of Health grant RO1AI049294 (to M. S.).
HEPES buffer | Fisher | BP310-500 | |
Sodium chloride | Fisher | BP358-212 | |
Magnesium chloride | Fisher | BP214-500 | |
Adenosine triphosphate (ATP) | Fisher | BP41325 | |
96-well plates (clear, flat-bottom) | VWR | 82050-760 | |
BIOMOL Green | Enzo Life Sciences | BML-AK111 | Preferred phosphate detection reagent. Caution: irritant. |
Microplate reader | BioTek Synergy or comparable | ||
Prism 5 | GraphPad Software |