Background: Engineered variants of the phytochrome photoreceptor are infrared fluorescent proteins. Results: Based on crystal structures, side chain substitutions near the chromophore were combined with monomerization of truncated phytochrome to yield an enhanced fluorophore. Conclusion: Amino acid changes that increase fluorescence discourage photoproduct formation. Significance: This improved infrared phytofluor provides long-wavelength excitation for high signal to noise in tissue and whole animals.

PilT is a hexameric ATPase required for bacterial type IV pilus retraction and surface motility. Crystal structures of ADP- and ATP-bound Aquifex aeolicus PilT at 2.8 and 3.2 A resolution show N-terminal PAS-like and C-terminal RecA-like ATPase domains followed by a set of short C-terminal helices. The hexamer is formed by extensive polar subunit interactions between the ATPase core of one monomer and the N-terminal domain of the next. An additional structure captures a nonsymmetric PilT hexamer in which approach of invariant arginines from two subunits to the bound nucleotide forms an enzymatically competent active site. A panel of pilT mutations highlights the importance of the arginines, the PAS-like domain, the polar subunit interface, and the C-terminal helices for retraction. We present a model for ATP binding leading to dramatic PilT domain motions, engagement of the arginine wire, and subunit communication in this hexameric motor. Our conclusions apply to the entire type II/IV secretion ATPase family.

Summary Proper formation of protein phosphatase 2A (PP2A) holoenzymes is essential for fitness of all eukaryotic cells. Carboxyl-methylation of PP2A catalytic subunit plays a critical role in regulating holoenzyme assembly; methylation is catalyzed by PP2A-specific methyltransferase LCMT-1, an enzyme required for cell survival. We determined crystal structures of human LCMT-1 in isolation and in complex with PP2A stabilized by a cofactor-mimic. The structures show that LCMT-1 active site pocket recognizes the carboxyl-terminus of PP2A, and interestingly, PP2A active site makes extensive contacts to LCMT-1. We demonstrated that activation of PP2A active site stimulates methylation, suggesting a mechanism for efficient conversion of activated PP2A into substrate-specific holoenzymes, thus minimizing unregulated phosphatase activity or formation of inactive holoenzymes. A dominant-negative LCMT-1 mutant attenuates cell cycle without causing cell death, likely by inhibiting uncontrolled phosphatase activity. Our studies suggested mechanisms of LCMT-1 in tight control of PP2A function, important for cell cycle and survival.

The 49-residue functional upstream domain (FUD) of Streptococcus pyogenes F1 adhesin interacts with fibronectin (FN) in a heretofore unknown manner that prevents assembly of a FN matrix. Biotinylated FUD (b-FUD) bound to adsorbed FN or its recombinant N-terminal 70-kDa fibrin- and gelatin-binding fragment (70K). Binding was blocked by FN or 70K, but not by fibrin- or gelatin-binding subfragments of 70K. Isothermal titration calorimetry showed that FUD binds with Kd values of 5.2 and 59 nm to soluble 70K and FN, respectively. We tested sets of FUD mutants and epitope-mapped monoclonal antibodies (mAbs) for ability to compete with b-FUD for binding to FN or to block FN assembly by cultured fibroblasts. Deletions or alanine substitutions throughout FUD caused loss of both activities. mAb 4D1 to the 2FNI module had little effect, whereas mAb 7D5 to the 4FNI module in the fibrin-binding region, 5C3 to the 9FNI module in the gelatin-binding region, or L8 to the G-strand of 1FNIII module adjacent to 9FNI caused loss of binding of b-FUD to FN and decreased FN assembly. Conversely, FUD blocked binding of 7D5, 5C3, or L8, but not of 4D1, to FN. Circular dichroism indicated that FUD binds to 70K by β-strand addition, a possibility supported by modeling based on crystal structures of peptides bound to 2FNI-5FNI of the fibrin-binding domain and 8FNI-9FNI of the gelatin-binding domain. Thus, the interaction likely involves an extensive anti-parallel β-zipper in which FUD interacts with the E-strands of 2FNI-5FNI and 8FNI-9FNI.

Preorganization is shown to endow a protein with extraordinary conformational stability. This preorganization is achieved by installing side-chain substituents that impose stereoelectronic and steric effects that restrict main-chain torsion angles. Replacing proline residues in ðProProGlyÞ 7 collagen strands with 4-fluoroproline and 4-methylproline leads to the most stable known triple helices, having T m values that are increased by >50°C. Differential scanning calorimetry data indicate an entropic basis to the hyperstability, as expected from an origin in preorganization. Structural data at a resolution of 1.21 Å reveal a prototypical triple helix with insignificant deviations to its main chain, even though 2∕3 of the residues are nonnatural. Thus, preorganization of a main chain by subtle changes to side chains can confer extraordinary conformational stability upon a protein without perturbing its structure.collagen triple helix | nonnatural amino acid | preorganization | protein stability | x-ray crystallography T he three-dimensional structures of proteins are stable because of a delicate balance of forces (1). Increases in the conformational stability of a protein-desirable in many contexts (2)-can be realized by enhancing the hydrophobic effect (3, 4), introducing or shielding a hydrogen bond (5) or electrostatic interaction (6-8), or introducing a disulfide crosslink (9-11) or metal ion-binding site (12). These strategies are often frustrated by enthalpy-entropy compensation, whereby enthalpic stabilization is offset entirely by entropic destabilization, or vice versa (13). Moreover, the strategies often lack subtlety and can lead to a misshapen and hence dysfunctional protein.As elaborated by Cram (14), the principle of preorganization states that, "the more highly hosts and guests are organized for binding and low solvation prior to their complexation, the more stable will be their complexes." In a typical manifestation, a conformational constraint is imposed upon a receptor or ligand during its chemical synthesis. This principle can also yield biopolymers with increased conformational stability (Fig. 1). For example, the stability of a DNA duplex correlates with the helicity of its single strands (15, 16), and a bicyclic ("locked") derivative of ribose enhances that stability (17, 18). The stability of β-turns within protein structures can be increased by incorporating Dproline (19) and certain β-amino acids (20) that bias the conformations occupied by the unfolded polypeptide. Likewise, proteins containing a cis-peptide bond can be stabilized by constrained amides or isosteres (21-23). The utility of these substitutions is limited, however, by the difficulty of modifying the backbone of proteins as well as concomitant effects on structure and function (24-27).We suspected that alterations to proline residues could provide a particularly incisive means to preorganize the conformation of a polypeptide chain. In classic work, Matthews and coworkers substituted proline, which is the least flexible residu...

The structure of the long-chain flavodoxin from the photosynthetic cyanobacterium Anabaena 7120 has been determined at 2 A resolution by the molecular replacement method using the atomic coordinates of the long-chain flavodoxin from Anacystis nidulans. The structure of a third long-chain flavodoxin from Chondrus crispus has recently been reported. Crystals of oxidized A . 7120 flavodoxin belong to the monoclinic space group P2, with a = 48.0, b = 32.0, c = 51.6A, and 0 = 92", and one molecule in the asymmetric unit. The 2 A intensity data were collected with oscillation films at the CHESS synchrotron source and processed to yield 9,795 independent intensities with Rmerg of 0.07. Of these, 8,493 reflections had I > 2a and were used in the analysis. The model obtained by molecular replacement was initially refined by simulated annealing using the XPLOR program. Repeated refitting into omit maps and several rounds of conjugate gradient refinement led to an R-value of 0.185 for a model containing atoms for protein residues 2-169, flavin mononucleotide (FMN), and 104 solvent molecules. The FMN shows many interactions with the protein with the isoalloxazine ring, ribityl sugar, and the 5'-phosphate. The flavin ring has its pyrimidine end buried into the protein, and the functional dimethyl benzene edge is accessible to solvent. The FMN interactions in all three long-chain structures are similar except for the 04' of the ribityl chain, which interacts with the hydroxyl group of Thr 88 side chain in A. 7120, while with a water molecule in the other two. The phosphate group interacts with the atoms of the 9-15 loop as well as with NE1 of Trp 57. The N5 atom of flavin interacts with the amide NH of Ile 59 in A . 7120, whereas in A . nidulans it interacts with the amide NH of Val 59 in a similar manner. In C. crispus flavodoxin, N5 forms a hydrogen bond with the side chain hydroxyl group of the equivalent Thr 58. The hydrogen bond distances to the backbone NH groups in the first two flavodoxins are 3.6 A and 3.5 A , respectively, whereas in the third flavodoxin the distance is 3.1 A , close to the normal value. Even though the hydrogen bond distances are long in the first two cases, still they might have significant energy because their microenvironment in the protein is not accessible to solvent. In all three long-chain flavodoxins, a water molecule bridges the ends of the inserted loop in the os strand and minimally perturbs its hydrogen bonding with p4. Many of the water molecules in these proteins interact with the flavin binding loops. The conserved &core of the three long-chain and two short-chain flavodoxins superpose with root mean square deviations ranging from 0.48 A to 0.97 A .

The catalytic subunit of protein phosphatase 2A (PP2Ac) is stabilized in a latent form by α4, a regulatory protein essential for cell survival and biogenesis of all PP2A complexes. Here we report the structure of α4 bound to the N-terminal fragment of PP2Ac. This structure suggests that α4 binding to the full-length PP2Ac requires local unfolding near the active site, which perturbs the scaffold subunit binding site at the opposite surface via allosteric relay. These changes stabilize an inactive conformation of PP2Ac and convert oligomeric PP2A complexes to the α4 complex upon perturbation of the active site. The PP2Ac–α4 interface is essential for cell survival and sterically hinders a PP2A ubiquitination site, important for the stability of cellular PP2Ac. Our results show that α4 is a scavenger chaperone that binds to and stabilizes partially folded PP2Ac for stable latency, and reveal a mechanism by which α4 regulates cell survival, and biogenesis and surveillance of PP2A holoenzymes.

Type IV pili are bacterial extracellular filaments that can be retracted to create force and motility. The retraction is accomplished by the motor protein PilT. Crystal structures of Pseudomonas aeruginosa PilT with and without bound AMP-PCP have been solved at 2.6 and 3.1 Å resolution, respectively, revealing an interlocking hexamer formed by the action of a crystallographic 2-fold symmetry operator on three subunits in the asymmetric unit and held together with extensive ionic interactions. The roles of two invariant carboxylates, Asp Box motif Glu163 and Walker B motif Glu204, have been assigned to Mg2+ binding and catalysis, respectively. The nucleotide ligands in each of the subunits in the asymmetric unit of the AMP-PCP bound PilT are not equally well ordered. Similarly, the three subunits in the asymmetric unit of both structures exhibit differing relative conformations of the two domains. The 12° and 20° domain rotations indicate motions that occur during the ATP-coupled mechanism of disassembly of pili into membrane-localized pilin monomers. Integrating these observations, we propose a three-state Ready, Active, Release model for the action of PilT.