Sensor phosphorylation

Figure 1.2 Structures of (a) tryptophan-based and (b) fluorescein-based kinase activity sensors. Phosphorylated residue (Ser) highlighted for clarity.

In enteric bacteria, expression of the nif genes is under general nitrogen regulation by the ntr system. This system consists of essentially a sensor-activator pair of proteins, encoded by ntrB and ntr(d. When fixed nitrogen is limiting, the sensor, NtrB, phosphorylates (and so activates) the activator,... 

There are no new messengers which act as cell-membrane transmitters in plants except cytokinins calcium is more widely used than in unicellular organisms but much less so than in animals. Plants respond to light via phosphorylation and changes from dormancy requires die change of cell calcium. Response times >1 s. There are several other sensors which are sometimes described as hormones, e.g. glucose and NO. 

One way to reduce the number of independent variables in the FRET-adjusted spectral equation is to use samples with a fixed donor-to-acceptor ratio. Under these conditions, the values of d and a are no longer independent, but rather the concentration of d is now a function of a and vice-versa. This approach is typical for the situation of FRET-based biosensor constructs. These sensors normally are designed to have a donor fluorophore attached to an acceptor by a domain whose structure is altered either as a result of a biological activity (such as proteolysis or phosphorylation), or by its interaction with a specific ligand with which it has high affinity. In general, FRET based biosensors have a stoichiometry of one... 

The two-component pathway is characterized by two functional elements. A histidine-specific protein kinase functions as a sensor that registers an external signal and passes this on to a downstream response regulator. The latter is activated by phosphorylation during the process of signal transduction, triggering other reactions in the cell (Fig. 12.3). 

Fig. 12.3. Principle of the two-component pathway. The fignre shows the principal steps of the two-component pathway in bacterial systems. An extraceUnlar signal (change in osmolarity, N availability, etc.) is registered by a receptor. An interaction takes place with the first component, the sensor kinase", which undergoes autophosphorylation at a His residue (H). The phosphate residue is transferred to the carboxyl side chain of an Asp residue (D) of the reaction regulator. Phosphorylation of the second component activates this for further signal conduction. The sen-sor kinase" may also be localized in the cytoplasmic domain of the receptor.

Fig. 12.4. Example of a two-component pathway in S. cerevisiae. Model of signal transdnction via the SLNl protein. The SLNl protein is a transmembrane protein with two transmembrane elements, which is assumed to exist as a dimer. The sensor domain and the regulator domain are localized on the same protein chain in the SLNl protein. The SLNl protein is activated by an extracellular signal (e.g., decrease in osmolarity). Autophosphorylation takes place on His (H) in the sensor domain and on Asp (D) in the regulator domain. A phosphate transfer takes place from the phosphohisti-dine to the effector protein SSKl. In the unphosphory-lated form, SSKl activates a MAPK pathway, which contains the protein kinase HOGl as a MAPK element. Various cellular reactions are triggered by HOGL If SSKl is phosphorylated in the course of activation of the two-component pathway, stimulation of the MAPK pathway is stopped. According to Swanson et al., (1994).

TorR -I- ATP <103> (<103>, TorS is a sensor that contains three phosphorylation sites and transphosphorylates TorR via a four-step phosphorelay, His443 to Asp723 to His850 to Asp(TorR). TorS can dephosphorylate phospho-TorR when trimethylamine N-oxide is removed. Dephosphorylation probably occurs by a reverse phosphorelay, Asp(TorR) to His850 to Asp723 [167]) (Reversibility <103> [167]) [167]... 

Jourlin, C. Ansaldi, M. Mejean, V. Transphosphorylation of the TorR response regulator requires the three phosphorylation sites of the TorS unorthodox sensor in Escherichia coli. J. Mol. Biol., 267, 770-777 (1997)... 

In addition to kinase-phosphatase cycles, bacteria use at least two other ATP-dependent regulatory mechanisms.110 In the sensor kinase/response regulator (or "two-component") systems110 111 a sensor protein, upon being allosterically activated, phosphorylates itself (autophosphorylation) on a specific histidyl residue to form an N-phosphohistidine derivative. 

The sensor protein is NtrB (NRn), which is an auto-phosphorylating histidine kinase similar to the CheY protein of bacterial chemotaxis (Fig. 19-5). 

Lonart G, Schoch S, KaeserPS et al (2003) Phosphorylation of RIMlalphaby PKA triggers presynaptic long-term potentiation at cerebellar parallel fiber synapses. Cell 115 49-60 Lou X, Scheuss V, Schneggenburger R (2005) Allosteric modulation of the presynaptic Ca2+ sensor for vesicle fusion. Nature 435 497-501... 

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