Synthesis of caged peptides using caged lysine: application to the synthesis of caged AIP, a highly specific inhibitor of calmodulin-dependent protein kinase II

Triggering biological processes: methods and applications of photocaged peptides and proteins

There has been a significant push in recent years to deploy fundamental knowledge and methods of photochemistry toward biological ends. Photoreactive groups have enabled chemists to activate biological function using the concept of photocaging. By granting spatiotemporal control over protein activation, these photocaging methods are fundamental in understanding biological processes. Peptides and proteins are an important group of photocaging targets that present conceptual and technical challenges, requiring precise chemoselectivity in complex polyfunctional environments. This review focuses on recent advances in photocaging techniques and methodologies, as well as their use in living systems. Photocaging methods include genetic and chemical approaches that require a deep understanding of structure-function relationships based on subtle changes in primary structure. Successful implementation of these ideas can shed light on important spatiotemporal aspects of living systems.

A caged substrate peptide for matrix metalloproteinases

Based on the widely applied fluorogenic peptide FS-6 (Mca-Lys-Pro-Leu-Gly-Leu-Dpa-Ala-Arg-NH2; Mca = methoxycoumarin-4-acetyl; Dpa = N-3-(2,4-dinitrophenyl)l-α,β-diaminopropionyl) a caged substrate peptide Ac-Lys-Pro-Leu-Gly-Lys*-Lys-Ala-Arg-NH2 (*, position of the cage group) for matrix metalloproteinases was synthesized and characterized. The synthesis implies the modification of a carbamidated lysine side-chain amine with a photocleavable 2-nitrobenzyl group. Mass spectrometry upon UV irradiation demonstrated the complete photolytic cleavage of the protecting group. Time-resolved laser-flash photolysis at 355 nm in combination with transient absorption spectroscopy determined the biphasic decomposition with τa = 171 ± 3 ms (79%) and τb = 2.9 ± 0.2 ms (21%) at pH 6.0 of the photo induced release of the 2-nitrobenzyl group. The recombinantly expressed catalytic domain of human membrane type I matrix metalloproteinase (MT1-MMP or MMP-14) was used to determine the hydrolysis efficiency of the caged peptide before and after photolysis. It turned out that the cage group sufficiently shields the peptide from peptidase activity, which can be thus controlled by UV light.

Chemical tools for studying directed cell migration

Cell migration is required for many physiological processes, including wound repair and embryogenesis, and relies on precisely orchestrated events that are regulated in a spatially and temporally controlled manner. Most traditional approaches for studying migration, such as genetic methods or the use of chemical inhibitors, do not offer insight into these important components of protein function. However, chemical tools, which respond on a more rapid time scale and in localized regions of the cell, are capable of providing more detailed, real-time information. This Review describes these recent approaches to investigate cell migration and focuses on proteins that are activated by light or small molecules, as well as fluorescent sensors of protein activity.

Light-mediated remote control of signaling pathways

Cell signaling networks display an extraordinary range of temporal and spatial plasticity. Our programmatic approach focuses on the construction of intracellular probes, including sensors, inhibitors, and functionally unique proteins that can be temporally and spatially controlled by the investigator even after they have entered the cell. We have designed and evaluated protein kinase sensors that furnish a fluorescent readout upon phosphorylation. In addition, since the sensors are inert (i.e., cannot be phosphorylated) until activated by light, they can be carried through the various stages of any given cell-based behavior without being consumed. Using this strategy, we have shown that PKCbeta is essential for nuclear envelope breakdown and thus the transition from prophase to metaphase in actively dividing cells. Photoactivatable proteins furnish the means to initiate cellular signaling pathways with a high degree of spatial and temporal control. We have used this approach to demonstrate that cofilin serves as a component of the steering apparatus of the cell. Finally, inhibitors are commonly used to assess the participation of specific enzymes in signaling pathways that control cellular behavior. We have constructed a photo-deactivatable inhibitor, an inhibitory species that can be switched off with light. In the absence of light, the target enzyme is inactive due to the presence of the potent inhibitory molecule. Upon photolysis, the inhibitory molecule is destroyed and enzymatic activity is released.

Illuminating the chemistry of life: design, synthesis, and applications of "caged" and related photoresponsive compounds

Biological systems are characterized by a level of spatial and temporal organization that often lies beyond the grasp of present day methods. Light-modulated bioreagents, including analogs of low molecular weight compounds, peptides, proteins, and nucleic acids, represent a compelling strategy to probe, perturb, or sample biological phenomena with the requisite control to address many of these organizational complexities. Although this technology has created considerable excitement in the chemical community, its application to biological questions has been relatively limited. We describe the challenges associated with the design, synthesis, and use of light-responsive bioreagents; the scope and limitations associated with the instrumentation required for their application; and recent chemical and biological advances in this field.

Caged molecules: principles and practical considerations

A caged molecule is an inert but photosensitive molecule that is transformed by photolysis into a biologically active molecule at high speed (typically 1 msec). The process is referred to as photorelease. The spatial resolution of photorelease is limited by the properties of light; submicrometer resolution is potentially achievable. Therefore, focal photorelease of caged molecules enables one to control biological processes with high spatio-temporal precision. The principles underlying caged molecules as well as practical considerations for their use are discussed in this unit.

Development of photolabile caged analogs of endothelin-1

Photoactivable caged analogs of endothelin-1 (ET-1) were obtained after derivatization with the photolabile 4,5-dimethoxynitrobenzyl (DMNB) group. This was achieved by the incorporation of N-alpha-Fmoc caged building blocks of Lys, Asp, Glu and Tyr during the solid phase peptide synthesis step. The C-terminal carboxylic function was also derivatized. However, difficulties were encountered with the introduction of the Asp and Glu photoactivable building blocks. As a matter of fact, formation of an aminosuccinyl derivative, through cyclization of the Asp(ODMNB) residue, and the formation of a pyrrolidone ring from the Glu(ODMNB) residue were highly favored by the electronic properties of the photocleavable function. ET-1 analogs were also tested in the ET(A) and ET(B) paradigms and specific pharmacological profiles were obtained for each peptide.

Photo-control of kinesin-microtubule motility using caged peptides derived from the kinesin C-terminus domain

To design a nanoscale biodevice that can be controlled by an external stimulus, we have introduced photochemical switching peptides derived from the kinesin C-terminus domain into the kinesin-microtubule in vitro motility system.

Development and pharmacological characterization of "caged" urotensin II analogs

Urotensin-II (U-II) is a cyclic 11-amino acid peptide known as a potent mammalian vasoconstrictor. To study some purported intracellular actions of U-II, masked analogs of this peptide, becoming biologically active only upon UV exposure, were developed. Those analogs described as "caged" were derivatized with a photolabile 4,5-dimethoxynitrobenzyl group on the side chain of Lys-8 or Tyr-9. Both caged analogs of U-II showed a major decrease in their affinity towards the UT receptor. Nevertheless, upon UV irradiation, the native and biologically active U-II peptide was recovered. Thus, this work describes the development of new "caged" U-II derivatives and demonstrates that vasoactivity of U-II can be controlled by masking and unmasking two key residues.

Caged RNase: photoactivation of the enzyme from perfect off-state by site-specific incorporation of 2-nitrobenzyl moiety

Photo-triggered activation of semisynthetic Ribonuclease S' from a perfect off-state was successfully carried out by incorporation of photo-labile caged moiety into a proximity to the active site.

A caged sperm-activating peptide that has a photocleavable protecting group on the backbone amide

A backbone-caged sperm-activating peptide (caged speract) that has a 2-nitrobenzyl group at a backbone amide and a vastly reduced affinity for its receptor (IC50=950 nM) was synthesized. UV irradiation of caged speract photocleaves the 2-nitrobenzyl group (tau1/2=26 micros), restoring its affinity (IC50=0.67 nM) and ability to increase sperm intracellular pH and Ca2+, as intact speract. Backbone caging of the biological activity was more efficient than side chain caging, which adds a nitrobenzyl group on the peptide side chain. The backbone caging strategy described can be used as a general procedure to cage biologically active peptides, which have no side chain for introduction of a caging group.

Real time visualization of protein kinase activity in living cells

A library of fluorescently labeled protein kinase C (PKC) peptide substrates was prepared to identify a phosphorylation-induced reporter of protein kinase activity. The lead PKC substrate displays a 2.5-fold change in fluorescence intensity upon phosphorylation. PKC activity is readily sampled in cell lysates containing the activated PKCs. Immunodepletion of conventional PKCs from the cell lysate eliminates the fluorescence response, suggesting that this peptide substrate is selectively phosphorylated by PKCalpha, beta, and gamma. Finally, living cells microinjected with the peptide substrate exhibit a 2-fold increase in fluorescence intensity upon exposure to a PKC activator. These results suggest that peptide-based protein kinase biosensors may be useful in monitoring the temporal and spatial dynamics of PKC activity in living cells.

Kinetic characterization of ribonuclease S mutants containing photoisomerizable phenylazophenylalanine residues

Incorporation of the photoisomerizable amino acid phenylazophenylalanine (PAP) into enzyme structures has been proposed as a strategy for photoswitching enzyme activity. To evaluate the strengths and limitations of this approach to enzyme photo-control, we performed a kinetic analysis of RNase S analogues containing PAP in positions 4, 7, 8, 10, 11 or 13. For an enzyme containing a single PAP group, the maximum extent of photoconversion (between approximately 96% trans/4% cis and 10% trans/90% cis under standard conditions) sets a limit on the maximum fold change in the initial rate of approximately 25-fold, if the cis form is the more active isomer, and approximately 10-fold if the trans form is more active. This extent of photoswitching was not realized in the present case because the effects of photoisomerization on kinetic constants were small and distributed among effects on S-peptide binding, substrate binding and the rate of the chemical step. These results suggest that photoisomerization could substantially alter enzyme kinetic constants but that a directed combinatorial approach might be required for realizing maximal photo-control in such systems. The limit set by the extent of photoconversion might be overcome by coupling multiple PAP groups to one enzyme or by altering the behaviour of a system that required oligomerization for activity.

Synthesis and application of caged peptides and proteins

Caged compounds have covalently attached groups that are rapidly cleaved upon exposure to UV light. Attachment of photolabile groups makes the molecule inert until photolysis releases it in its bioactive form. When caged compounds are applied to the experimental system in advance, the concentration jump of biologically active substances can be brought about immediately in a limited area upon irradiation with pulsed and focused UV light. Therefore, caged compounds of low molecular weight, which are commercially available, have been used effectively to study the mechanisms of temporal biological phenomena, such as muscle contraction, intracellular signaling, and neurotransmission. Because many proteins and peptides play important roles in these phenomena, their caged derivatives should serve as powerful tools to clarify complex biological systems. To prepare caged proteins and peptides, several groups have improved upon a chemical modification method, as well as developed two new methods: (1) nonsense codon suppression and (2) solid-phase peptide synthesis. In this review, we summarize recent advances made in the design, preparation, and application of caged peptides and proteins.

Regulation of nuclear import by light-induced activation of caged nuclear localization signal in living cells

A novel fluorescence probe suitable for the study of nuclear import in living cells has been developed. The lysine-128 residue in SV40 T-antigen nuclear localization signal (NLS) was converted to a caged lysine with the amino acid blocked by a photocleavable protecting group. Following irradiation of ultraviolet (UV) light, the caged NLS conjugate translocated into and accumulated in the nucleus within 20 min similar to uncaged NLS conjugate. Maximum import rate saturated approximately 4.78+/-0.21% per minute when the duration of irradiation was more than 1/15 s (22 mW/cm(2)). Caged NLS conjugate tended to distribute near the surface of the nucleus, and this association became stronger after UV irradiation. The caged conjugate enabled us to regulate the initial state of the reaction, both spatially and temporally.