Introduction of a tryptophan reporter group into the ATP binding motif of the Escherichia coli UvrB protein for the study of nucleotide binding and conformational dynamics

Photoradical-Mediated Catalyst-Independent Protein Cross-Link with Unusual Fluorescence Properties

Photo-actively modified natural amino acids have served as lucrative probes for precise mapping of the dynamics, interaction networks, and turnover of cytosolic proteins both in vivo and ex vivo. In our attempts to extend the utility of photoreactive reporters to map the molecular characteristics of vital membrane proteins, we carried out site-selective incorporation of 7-fluoro-indole in the human mitochondrial outer membrane protein VDAC2 (voltage-dependent anion channel isoform 2), with the aim of generating Trp-Phe/Tyr cross-links. Prolonged irradiation at 282 nm provided us with a surprisingly unusual fluorophore that displayed sizably red-shifted excitation (λex-max =280 nm→360 nm) and emission (λem-max =330 nm→430 nm) spectra that was reversible with organic solvents. By measuring the kinetics of the photo-activated cross-linking with a library of hVDAC2 variants, we demonstrate that formation of this unusual fluorophore is kinetically retarded, independent of tryptophan, and is site-specific. Using other membrane (Tom40 and Sam50) and cytosolic (MscR and DNA Pol I) proteins, we additionally show that formation of this fluorophore is protein-independent. Our findings reveal the photoradical-mediated accumulation of reversible tyrosine cross-links, with unusual fluorescent properties. Our findings have immediate applications in protein biochemistry and UV-mediated protein aggregation and cellular damage, opening avenues for formulating therapeutics that prolong cell viability in humans.

Divalent Metal Ions Mg²⁺ and Ca²⁺ Have Distinct Effects on Protein Kinase A Activity and Regulation

cAMP-dependent protein kinase (PKA) is regulated primarily in response to physiological signals while nucleotides and metals may provide fine-tuning. PKA can use different metal ions for phosphoryl transfer, yet some, like Ca(2+), do not support steady-state catalysis. Fluorescence Polarization (FP) and Surface Plasmon Resonance (SPR) were used to study inhibitor and substrate interactions with PKA. The data illustrate how metals can act differentially as a result of their inherent coordination properties. We found that Ca(2+), in contrast to Mg(2+), does not induce high-affinity binding of PKA to pseudosubstrate inhibitors. However, Ca(2+) works in a single turnover mode to allow for phosphoryl-transfer. Using a novel SPR approach, we were able to directly monitor the interaction of PKA with a substrate in the presence of Mg(2+)ATP. This allows us to depict the entire kinase reaction including complex formation as well as release of the phosphorylated substrate. In contrast to Mg(2+), Ca(2+) apparently slows down the enzymatic reaction. A focus on individual reaction steps revealed that Ca(2+) is not as efficient as Mg(2+) in stabilizing the enzyme:substrate complex. The opposite holds true for product dissociation where Mg(2+) easily releases the phospho-substrate while Ca(2+) traps both reaction products at the active site. This explains the low steady-state activity in the presence of Ca(2+). Furthermore, Ca(2+) is able to modulate kinase activity as well as inhibitor binding even in the presence of Mg(2+). We therefore hypothesize that the physiological metal ions Mg(2+) and Ca(2+) both play a role in kinase activity and regulation. Since PKA is localized close to calcium channels and may render PKA activity susceptible to Ca(2+), our data provide a possible mechanism for novel crosstalk between cAMP and calcium signaling.

UvrB domain 4, an autoinhibitory gate for regulation of DNA binding and ATPase activity

UvrB, a central DNA damage recognition protein in bacterial nucleotide excision repair, has weak affinity for DNA, and its ATPase activity is activated by UvrA and damaged DNA. Regulation of DNA binding and ATP hydrolysis by UvrB is poorly understood. Using atomic force microscopy and biochemical assays, we found that truncation of domain 4 of Bacillus caldotenax UvrB (UvrBDelta4) leads to multiple changes in protein function. Protein dimerization decreases with an approximately 8-fold increase of the equilibrium dissociation constant and an increase in DNA binding. Loss of domain 4 causes the DNA binding mode of UvrB to change from dimer to monomer, and affinity increases with the apparent dissociation constants on nondamaged and damaged single-stranded DNA decreasing 22- and 14-fold, respectively. ATPase activity by UvrBDelta4 increases 14- and 9-fold with and without single-stranded DNA, respectively, and UvrBDelta4 supports UvrA-independent damage-specific incision by Cho on a bubble DNA substrate. We propose that other than its previously discovered role in regulating protein-protein interactions, domain 4 is an autoinhibitory domain regulating the DNA binding and ATPase activities of UvrB.

The presence of two UvrB subunits in the UvrAB complex ensures damage detection in both DNA strands

It is generally accepted that the damage recognition complex of nucleotide excision repair in Escherichia coli consists of two UvrA and one UvrB molecule, and that in the preincision complex UvrB binds to the damage as a monomer. Using scanning force microscopy, we show here that the damage recognition complex consists of two UvrA and two UvrB subunits, with the DNA wrapped around one of the UvrB monomers. Upon binding the damage and release of the UvrA subunits, UvrB remains a dimer in the preincision complex. After association with the UvrC protein, one of the UvrB monomers is released. We propose a model in which the presence of two UvrB subunits ensures damage recognition in both DNA strands. Upon binding of the UvrA(2)B(2) complex to a putative damaged site, the DNA wraps around one of the UvrB monomers, which will subsequently probe one of the DNA strands for the presence of a lesion. When no damage is found, the DNA will wrap around the second UvrB subunit, which will check the other strand for aberrations.

The nucleotide excision repair protein UvrB, a helicase-like enzyme with a catch

Nucleotide excision repair (NER) is a universal DNA repair mechanism found in all three kingdoms of life. Its ability to repair a broad range of DNA lesions sets NER apart from other repair mechanisms. NER systems recognize the damaged DNA strand and cleave it 3', then 5' to the lesion. After the oligonucleotide containing the lesion is removed, repair synthesis fills the resulting gap. UvrB is the central component of bacterial NER. It is directly involved in distinguishing damaged from undamaged DNA and guides the DNA from recognition to repair synthesis. Recently solved structures of UvrB from different organisms represent the first high-resolution view into bacterial NER. The structures provide detailed insight into the domain architecture of UvrB and, through comparison, suggest possible domain movements. The structure of UvrB consists of five domains. Domains 1a and 3 bind ATP at the inter-domain interface and share high structural similarity to helicases of superfamilies I and II. Not related to helicase structures, domains 2 and 4 are involved in interactions with either UvrA or UvrC, whereas domain 1b was implicated for DNA binding. The structures indicate that ATP binding and hydrolysis is associated with domain motions. UvrB's ATPase activity, however, is not coupled to the separation of long DNA duplexes as in helicases, but rather leads to the formation of the preincision complex with the damaged DNA substrate. The location of conserved residues and structural comparisons with helicase-DNA structures suggest how UvrB might bind to DNA. A model of the UvrB-DNA interaction in which a beta-hairpin of UvrB inserts between the DNA double strand has been proposed recently. This padlock model is developed further to suggest two distinct consequences of domain motion: in the UvrA(2)B-DNA complex, domain motions lead to translocation along the DNA, whereas in the tight UvrB-DNA pre-incision complex, they lead to distortion of the 3' incision site.

Crystal structure of the DNA nucleotide excision repair enzyme UvrB from Thermus thermophilus

Nucleotide excision repair (NER) is the most important DNA-repair mechanism in living organisms. In prokaryotes, three enzymes forming the UvrABC system initiate NER of a variety of structurally different DNA lesions. UvrB, the central component of this system, is responsible for the ultimate DNA damage recognition and participates in the incision of the damaged DNA strand. The crystal structure of Thermus thermophilus UvrB reveals a core that is structurally similar to core regions found in helicases, where they constitute molecular motors. Additional domains implicated in binding to DNA and various components of the NER system are attached to this central core. The architecture and distribution of DNA binding sites suggest a possible model for the DNA damage recognition process.

Oligomerization of the UvrB nucleotide excision repair protein of Escherichia coli

A combination of hydrodynamic and cross-linking studies were used to investigate self-assembly of the Escherichia coli DNA repair protein UvrB. Though the procession of steps leading to incision of DNA at sites flanking damage requires that UvrB engage in an ordered series of complexes, successively with UvrA, DNA, and UvrC, the potential for self-association had not yet been reported. Gel permeation chromatography, nondenaturing polyacrylamide gel electrophoresis, and chemical cross-linking results combine to show that UvrB stably assembles as a dimer in solution at concentrations in the low micromolar range. Smaller populations of higher order oligomeric species are also observed. Unlike the dimerization of UvrA, an initial step promoted by ATP binding, the monomer-dimer equilibrium for UvrB is unaffected by the presence of ATP. The insensitivity of cross-linking efficiency to a 10-fold variation in salt concentration further suggests that UvrB self-assembly is driven largely by hydrophobic interactions. Self-assembly is significantly weakened by proteolytic removal of the carboxyl terminus of the protein (generating UvrB*), a domain also known to be required for the interaction with UvrC leading to the initial incision of damaged DNA. This suggests that the C terminus may be a multifunctional binding domain, with specificity regulated by protein conformation.

Tryptophan fluorescence monitors multiple conformational changes required for glutamine phosphoribosylpyrophosphate amidotransferase interdomain signaling and catalysis

Single tryptophan residues were incorporated into each of three peptide segments that play key roles in the structural transition of ligand-free, inactive glutamine phosphoribosylpyrophosphate (PRPP) amidotransferase to the active enzyme-substrate complex. Intrinsic tryptophan fluorescence and fluorescence quenching were used to monitor changes in a phosphoribosyltransferase (PRTase) "flexible loop", a "glutamine loop", and a C-terminal helix. Steady state fluorescence changes resulting from substrate binding were used to calculate binding constants and to detect the structural rearrangements that coordinate reactions at active sites for glutamine hydrolysis and PRTase catalysis. Pre-steady state kinetics of enzyme.PRPP and enzyme.PRPP.glutamine complex formation were determined from stopped-flow fluorescence measurements. The kinetics of the formation of the enzyme.PRPP complex were consistent with a model with two or more steps in which rapid equilibrium binding of PRPP is followed by a slow enzyme isomerization. This isomerization is ascribed to the closing of the PRTase flexible loop and is likely the rate-limiting step in the reaction of PRPP with NH(3). The pre-steady state kinetics for binding glutamine to the binary enzyme. PRPP complex could also be fit to a model involving rapid equilibrium binding of glutamine followed by an enzyme isomerization step. The changes monitored by fluorescence account for the interconversions between "end state" structures determined previously by X-ray crystallography and define an intermediate enzyme.PRPP conformer.