Studies of self-assembly of tobacco mosaic virus (TMV) are briefly reviewed and X-ray fiber diffraction analysis of TMV is described. The fiber diffraction analysis has enabled a molecular model of the intact virus to be built, based on a map at 3.6A resolution derived from five separated Bessel orders. This has been made possible by advances in the solution of the fiber diffraction phase problem. It is now possible to understand much of the chemical basis of TMV assembly, particularly in terms of inter-subunit electrostatic interactions and RNA binding. Consideration of the molecular structure in conjunction with physical chemical studies by other groups suggests that the nucleating aggregate for initiation of TMV assembly is not the 34-subunit disk, but a short (about two turns) helix of protein subunits, probably inhibited from further polymerization by the disordering of a peptide loop near the inner surface of the virus.
Maturation-promoting factor (MPF) is a cytoplasmic factor that is capable of inducing a maturation response involving nuclear envelope breakdown, chromosome condensation and spindle formation when microinjected into immature oocytes. MPF activity is found in a wide variety of eukaryotic cells at M-phase, such as mitotically dividing cells as well as maturing oocytes. MPF acts non-species-specifically among the animal kingdom. MPF has been partially purified and characterized as a heatlabile protein with a molecular size of approx. 5s. MPF activity oscillates with the same period as the cell cycle, with peaks during metaphase. MPF has the ability to amplify itself by activating its precursor, which is stored in fully grown oocytes. But replenishing MPF after its fall during cell cycle requires protein synthesis. MPF can cause nuclear envelope breakdown and chromosome condensation in vitro with isolated nuclei. Such cell-free system requires the cytoplasmic component extracted from eggs in addition to MPF. Further, MPF dissociates the junction between oocyte surface and surrounding follicle cells. Thus, MPF seems to act directly not on nucleus but on cytoplasm. Taken together, MPF might more generally be described as "metaphase-promoting factor" rather than a maturation-promoting factor.
Potential usefullness of infrared spectroscopy to direct measurement of the ligand binding to hemoprotein is discussed, refereing to CO and CN- binding. CO myoglobin has an asynmetric C-O stretch band which is decomvoluted into 4 symmetrical Gausian curves. The intensy of these decomvoluted bands depends on pH and temperature without any change in band position and half band width, indicating that myoglobin has 4 discrete and rapidly interconvertible conformers. Therefore, X-ray crystallographic structure is regarded as an average of the tertially sturucture of the two dominant conformers which give about 90% of the intergrated area of the experimental curve. C-N stretch bands of cytochrome oxidase cyanide show that cyanide binds only to Cu2+ at fully oxidized state, not to Fe3+ in the O2 binding site, but it does to both Fe and Cu at any oxidation state other than fully oxidized state. The infrared parameters of CN- bound to one metal is quite insensitive to the oxidation state and the ligand binding of the other metal. These are highly unique information about the structure and function of the ligand binding sites, which only infrared spectroscopy can proVide at present.