Insights into the mechanism of heterodimerization from the 1H-NMR solution structure of the c-Myc-Max heterodimeric leucine zipper

The oncoprotein c-Myc (a member of the helix-loop-helix-leucine zipper (b-HLH-LZ) family of transcription factors) must heterodimerize with the b-HLH-LZ Max protein to bind DNA and activate transcription. It has been shown that the LZ domains of the c-Myc and Max proteins specifically form a heterodimeric LZ at 20 degreesC and neutral pH. This suggests that the LZ domains of the c-Myc and Max proteins are playing an important role in the heterodimerization of the corresponding gene products in vivo. Initially, to gain an insight into the energetics of heterodimerization, we studied the stability of N-terminal disulfide-linked versions of the c-Myc and Max homodimeric LZs and c-Myc-Max heterodimeric LZ by fitting the temperature-induced denaturation curves monitored by circular dichroism spectroscopy. The c-Myc LZ does not homodimerize (as previously reported) and the c-Myc-Max heterodimeric LZ is more stable than the Max homodimeric LZ at 20 degreesC and pH 7.0. In order to determine the critical interhelical interactions responsible for the molecular recognition between the c-Myc and Max LZs, the solution structure of the disulfide-linked c-Myc-Max heterodimeric LZ was solved by two-dimensional 1H-NMR techniques at 25 degreesC and pH 4.7. Both LZs are alpha-helical and the tertiary structure depicts the typical left-handed super-helical twist of a two-stranded parallel alpha-helical coiled-coil. A buried salt bridge involving a histidine on the Max LZ and two glutamate residues on the c-Myc LZ is observed at the interface of the heterodimeric LZ. A buried H-bond between an asparagine side-chain and a backbone carbonyl is also observed. Moreover, evidence for e-g interhelical salt bridges is reported. These specific interactions give insights into the preferential heterodimerization process of the two LZs. The low stabilities of the Max homodimeric LZ and the c-Myc-Max heterodimeric LZ as well as the specific interactions observed are discussed with regard to regulation of transcription in this family of transcription factors.

The monolayer technique as a tool to study the energetics of protein-protein interactions

In this paper, we explore the possibility of using the monolayer technique and hydrophobic homopolypeptides to study the energetics of protein stability. We have studied the stabilization of the bilayer state of poly-L-alanine in its alpha-helical conformation at the air-water interface by measuring compression and expansion surface pressure (II)-residual area (A) isotherms at 22 +/- 2 degrees C. The Gibbs free energy of stabilization per alanyl residue transferred from the water exposed state in the monolayer to the inside of the bilayer was calculated from the surface area of the hysteresis loops obtained during compression-expansion cycles performed during the monolayer to bilayer transition. Using atomic solvation parameters and the water accessible surface area per atom group for an alanyl residue in a standard alpha-helix, we have dissected the free energy of stabilization per alanyl residue into the change of solvation free energy (delta Gs) upon transfer from the water surface to the inside of the bilayer state, and the free energy associated to the formation of hydrophobic van der Waals interactions (delta GvdW) in the bilayer. We estimate a value of 25 +/- 4 cal/(mol A2) for the hydrophobic interaction, as defined by the sum of delta Gs and delta GvdW per unit of hydrophobic (aliphatic) accessible surface area in an alanyl residue.

Formation of parallel and antiparallel coiled-coils controlled by the relative positions of alanine residues in the hydrophobic core

The orientation of alpha-helical chains in two-stranded coiled-coils has been shown to be determined by the presence of favorable interchain electrostatic interactions. In this study, we used de novo designed 35-residue peptides to show that when interchain electrostatic interactions are not a factor in coiled-coil formation, the relative positions of Ala residues in the middle heptad can control the parallel or antiparallel orientation of alpha-helical chains in coiled-coils. The peptides formed four-stranded coiled-coils where the helices are either all-parallel or all-antiparallel with respect to their nearest neighbor. The common structural element in these four-stranded coiled-coils is an alternating pair of Ala and Leu residues (Ala-Leu-Ala-Leu) in each of the two planes in the middle heptad. These results indicate that both the relative positions of the Ala residues in the hydrophobic core and the interchain electrostatic interactions between charged residues in the e and g positions should be considered in designing coiled-coils with the desired number of strands in the multiple-stranded assembly. These design elements are also important in orienting functional groups or domains attached to the terminals ends of a coiled-coil carrier.

Preferential heterodimeric parallel coiled-coil formation by synthetic Max and c-Myc leucine zippers: a description of putative electrostatic interactions responsible for the specificity of heterodimerization

The oncoprotein c-Myc must heterodimerize with Max to bind DNA and perform its oncogenic activity. The c-Myc-Max heterodimer binds DNA through a basic helix-loop-helix leucine zipper (b-HLH-zip) motif and it is proposed that leucine zipper domains could, in concert with the HLH regions, provide the specificity and stability of the b-HLH-zip motif. In this context, we have synthesized the peptides corresponding to the leucine zipper domains of Max and c-Myc with a N-terminal Cys-Gly-Gly linker and studied their dimerization behavior using reversed-phase HPLC and CD spectroscopy. The preferential formation of a fully helical parallel c-Myc-Max heterodimeric coiled-coil was observed under air-oxidation and redox conditions at neutral pH. We show that the stability and the helicity of the disulfide-linked c-Myc-Max heterostranded coiled-coil is modulated by pH, with a maximum around pH 4.5, supporting the existence of stabilizing and specific interhelical electrostatic interactions. We present a molecular model of the c-Myc-Max heterostranded coiled-coil describing potential electrostatic interactions responsible for the specificity of the interaction, the main feature being putative buried electrostatic interactions between a histidine side-chain (in the Max leucine zipper) and two glutamic acid side-chains (in the c-Myc leucine zipper) at the heterodimer interface. This model is supported by the fact that the apparent pKa (as determined by [1H]-NMR spectroscopy) of this histidine side-chain at 25 degrees C is 0.42 (+/- 0.05) pKa units higher in the folded form than in the unfolded form. This indicates that the charged histidine side-chain contributes approximately 0.57 (+/- 0.07) kcal/mol (2.38 (+/- 0.30) kJ/mol) of stabilization free energy to the c-Myc-Max heterostranded coiled-coil through favorable electrostatic interaction.

Increased frequency conversion of Nd:YAG laser radiation with a variable-reflectivity mirror

The frequency-doubling efficiency of a Nd:YAG laser was increased by using a variable-reflectivity-mirror unstable resonator. At 5 Hz as much as 385 mJ of green light was obtained with an efficiency of 61%. The divergence of the converted light, which was 0.26 mrad at 0.5 Hz, close to the diffraction limit, increased to 0.69 mrad at 5 Hz.

Variable reflectivity unstable resonators for coherent laser radar emitters

Beam properties obtained from a TE CO(2) laser in a Cassegrain resonator using various graded reflectivity mirror couplers are compared. It is shown that, experimentally, Gaussian profiles maximize the far-field intensity while parabolic profiles yield more uniform near fields with a lower energy extraction. Both configurations appear suited for coherent ladar systems.

Large-size Gaussian mode in unstable resonators using Gaussian mirrors

Gaussian modes with large sections have been experimentally produced in Cassegrain resonators using Gaussian reflectivity convex couplers. The far field of the beam, which was coupled through a Gaussian coupler, was found to be free from secondary rings.

Optical guided-beam splitter: an experimental study

Measurements on the reflectance R of a guided-beam splitter have been performed. The fabrication method, based on the use of a thin wire as a mask, produces a groove with a parabolic thickness profile in a planar waveguide. The experimental results demonstrate that the theoretical expression R = tanh(2)delta describes well the observed phenomena, especially when the TM(0) mode is incident on the groove. Additional applications of this device for polarization analysis and mode selection are also discussed.

Focusing of laser beams by a sequence of irises

The properties of diffraction focusing systems composed of a series of irises, coaxially distributed along a laser beam, are analyzed. Based on diffracted-ray methods, complete design equations and optimization procedures are given. Focusing experiments on a He-Ne laser beam are reported with two types of focusing structures, and the results are found to agree completely with the theoretical predictions.

TEA-laser emission on the sequence bands of CO2

Laser emission on the sequence bands of CO2 has been achieved in a TEA amplifier using either a hot-CO 2 cell or a Michelson interferometer to prevent oscillation of the regular bands. It has been found that the insertion losses of the hot-CO2 filter severely limit the energy extraction efficiency on the sequence bands. With the Michelson arrangement, output energies of 140 mJ on the sequence P(17) have been produced in a 33-cm-long TEA amplifier. The relatively low efficiencies obtained suggest that generation of high-intensity laser beams at the sequence band frequencies would require the use of oscillator-amplifier combinations.

Simultaneous frequency stabilization and injection in a TEA-CO(2) oscillator

A novel technique that uses a single master oscillator to serve both the injection and the stabilization function in a TEA-CO(2) laser transmitter is described. The method, based on a two-polarization scheme, provides sufficient optical isolation between the TEA laser pulse and the probing laser beam to prevent anydamage or saturation of the controlling system. Reproducible and stable SLM operation in the MW power levels is achieved on a long term basis.