Determination of phosphorus-32 and calcium-45 in biological samples by Cerenkov and liquid scintillation counting

Autophosphorylation Is a Mechanism of Inhibition in Twitchin Kinase

Titin-like kinases are muscle-specific kinases that regulate mechanical sensing in the sarcomere. Twitchin kinase (TwcK) is the best-characterized member of this family, both structurally and enzymatically. TwcK activity is auto-inhibited by a dual intrasteric mechanism, in which N- and C-terminal tail extensions wrap around the kinase domain, blocking the hinge region, the ATP binding pocket and the peptide substrate binding groove. Physiologically, kinase activation is thought to occur by a stretch-induced displacement of the inhibitory tails from the kinase domain. Here, we now show that TwcK inhibits its catalysis even in the absence of regulatory tails, by undergoing auto-phosphorylation at mechanistically important elements of the kinase fold. Using mass spectrometry, site-directed mutagenesis and catalytic assays on recombinant samples, we identify residues T212, T301, T316 and T401 as primary auto-phosphorylation sites in TwcK in vitro. Taken together, our results suggest that residue T316, located in the peptide substrate binding P+1 loop, is the dominantly regulatory site in TwcK. Based on these findings, we conclude that TwcK is regulated through a triple-inhibitory mechanism consisting of phosphorylation and intrasteric blockage, which is responsive not only to mechanical cues but also to biochemical modulation. This implies that mechanically stretched conformations of TwcK do not necessarily correspond to catalytically active states, as previously postulated. This further suggests a phosphorylation-dependent desensitization of the TwcK-mediated mechanoresponse of the sarcomere in vivo.

Quantification of Bacterial Histidine Kinase Autophosphorylation Using a Nitrocellulose Binding Assay

We demonstrate a useful method for quantifying autophosphorylation of purified bacterial histidine kinases. Histidine kinases are known for their involvement in two-component signal transduction, a ubiquitous system through which bacteria sense and respond to environmental stimuli. Two-component signaling features autophosphorylation of a histidine kinase, followed by phosphotransfer to the receiver domain of a response regulator protein, which ultimately leads to an output response. Autophosphorylation of the histidine kinase is responsive to the presence of a cognate environmental stimulus, thereby giving bacteria a means to detect and respond to changes in the environment. Despite their importance in bacterial biology, histidine kinases remain poorly understood due to the inherent lability of phosphohistidine. Conventional methods for studying these proteins, such as SDS-PAGE autoradiography, have significant shortcomings. We have developed a nitrocellulose binding assay that can be used to characterize histidine kinases. The protocol for this assay is simple and easy to execute. Our method is higher throughput, less time-consuming, and offers a greater dynamic range than SDS-PAGE autoradiography.