Myc and Max function as a nucleoprotein complex.

The Myc family of oncoproteins are thought to regulate proliferation and differentiation in a wide variety of cell types. Recent studies show that Myc proteins form sequence-specific DNA-binding complexes with Max, a new member of the helix-loop-helix leucine zipper protein class. The properties of the Myc-Max complex suggest a mechanism for Myc's function in both normal and neoplastic cell behavior.     

The Transient Response and Impedance Locus of a Mobile Site Membrane

The kinetic properties of a mobile site membrane are analyzed theoretically and the results are compared with experimental data obtained from a simple model system. In the model system the measured response to a step change in electric current is a single relaxation process, in excellent agreement with the theoretically predicted behavior. To this relaxation process corresponds an anomalous capacitance whose properties are obtained by performing a frequency analysis of the system. By plotting the impedance locus of this mobile site membrane, the locus is shown to exhibit a 45° phase angle at high frequencies, a phenomenon which is generally associated with depletion layers.     

Ionic selectivity,saturation, and block in gramicidin a channels

Summary A theory, recently developed by Sandblom, Eisenman and Neher (1977) for the conductance of single gramicidin A channels predicts three limiting behaviors of the relation between conductance and salt concentration. These are: (i) a saturating behavior resembling a simple adsorption isotherm at medium and high concentrations, (ii) a decrease in conductance at the highest obtainable concentrations and (iii) deviations from the isotherm at very low concentrations. Features i and ii have been described before. Experimental evidence for point iii is given here. The new feature points towards interactions among ions in the channel at ionic concentrations as low as 1–10mm.Particular emphasis is given to the behavior at very low salt concentrations and the experimental problems encountered in this situation. In addition, mutual blocking effects among monovalent ions in symmetrical salt mixtures are characterized and found to be in satisfactory agreement with theoretical expectations, based upon the single salt conductance data presented here and zero-current potentials in salt mixtures to be described in a subsequent paper.     

Characterization of the five-fold Ca2+ binding site of satellite tobacco necrosis virus using Eu3+ luminescence spectroscopy: a marked size-selectivity among rare earth ions.

Satellite tobacco necrosis virus (STNV) is an icosahedral virus which contains three classes of Ca2+ binding site. One of these classes, a five-fold carbonyl site which is believed to exist in a Ca2+ channel, has been investigated using laser-induced Eu3+ luminescence spectroscopy. These twelve identical sites are rather rigid, as evidenced by the single narrow (full width at half-maximum is 6.5 cm-1) band observed at 579.58 nm in the 7F0----5D0 excitation spectrum of the Eu(3+)-STNV complex. Lifetimes of 270 microseconds in H2O and 1620 microseconds in D2O indicate that there are three water molecules bound to the Eu3+ at this site. Ligand field splitting of the 7F0----5D1 and 7F0----5D2 excitation spectra show that this site possesses fairly high symmetry (less than or equal to C5V). The Eu3+ complex of nitrilotriacetic acid was determined via titration to have a dissociation constant, Kd, of 20 +/- 2 nM; this value has been used in competition experiments to deduce that the virus site class binds Eu3+ with a Kd of 1.1 +/- 0.3 nM. This putative ion channel demonstrates remarkable size selectivity, with lanthanide affinities varying by more than one order of magnitude.