GTP-binding proteins are involved in a wide variety of cellular functions. One of the most recent findings is the implication of many GTP-binding proteins in the pathway of protein secretion. In the eukaryotic secretory pathway, proteins have to be sorted from each other at many steps, including the signal recognition and translocation across the endoplasmic reticulum membrane and vesicular transport between membrane-bounded organelles. I propose here that GTP-binding proteins play a critical role in these sorting events. Several models of their action which is driven by GDP-GTP exchange will be presented.
One of the profound changes in cellular morphology which occur during mitosis is a massive alteration in the organization of the microfilament cytoskeleton. These changes, together with other mitotic events including nuclear membrane breakdown, chromosome condensation, and formation of mitotic spindles, are all induced by a single molecular complex called MPF, maturation promoting factor. We have demonstrated that cdc2 kinase, a catalytic subunit of MPF, phosphorylates nonmuscle caldesmon both in vivo and in vitro, which causes caldesmon to dissociate from microfilaments during mitosis. Our results suggest that cdc2 kinase directly controls microfilament re-organization during mitosis because caldesmon inhibits actomyosin ATPase.
Vertebrate photoreceptor cells, rods and cones, are responsible for scotopic and photopic vision, respectively. Based on their spectral sensitivities, cones can be further classified into several types having different visual pigments. This is the origin of color vision. As compared with a vast amount of work on the visual transduction process in rods, little is known about this process in cones. I have investigated characteristic properties of chicken cone visual pigments to elucidate eventually the molecular basis of color vision. Recent studies on iodopsin, a chicken red-sensitive cone pigment, including its primary structure and functions (photoreactions and interaction with a G-protein) are summarized in this article.
Recently developed methods of small-angle neutron scattering for molecular biology research, in particular, nuclear spin contrast variation method will be discussed by comparison of conventional methods. The Small-Angle Neutron Scattering instrument (SANS-1) at the research reactor FRG-1 at the GKSS Research Center, Germany, has been equipped with a polarized target station for the measurement of polarized neutron scatterings, which is mainly dedicated to the use of nuclear spin contrast variation method. First experiments using SANS-I with apoferritin and deuterated large subunit of E. coli ribosomes have shown the feasibility of this method.
Viscosity behaviors of aqueous solutions of sodium polystyrenesulfonate (NaPSS) and of latex dispersions were investigated with utmost care. Substantial shear-thinning effect was noted. The reduced viscosity of NaPSS solution (extrapolated to zero shear rate) increased with decreasing polymer concentration but after passing through a maximum it decreased. Considering this maximum, the intrinsic viscosity was estimated. The exponent α of the relation, [η]=K Mα, thus obtained was not two as widely believed, but was found to be 1.2-1.6, which implies that the conformation of the ionic polymer at infinite dilution is not rod-like, which contradicts to the widely accepted view. The maximum in viscosity curve was also observed for dispersions of latex particles, which do not undergo conformational change. Hence, the characteristic behavior of the viscosity of ionic polymer solutions is concluded to be not due to the conformational change of the polymer. The most important factors in the viscosity are the distortion of the ionic cloud around the polymer and/or the electrostatic interaction between ionic solutes.
Anaphase chromosome separation is a result of two distinct processes; chromosome-to-pole movement, and pole-to-pole movement (spindle elongation). In vitro studies have shown that 1) the force required for spindle elongation is generated by mechanochemical enzymes which mediate the sliding apart of antiparallel microtubules; and 2) the activity of kinetochore is responsible for chromosome-to-pole movement, and chromosomes in vitro can move polewards by microtubule chromosomes in vitro can move polewards by microtubule depolymerization at the kinetochore.
All extant organisms are thought to be classified into three primary kingdoms, eubacteria, eukaryotes, and archaebacteria. Based on the analyses of a pair of duplicated genes, elongation factors EF-Tu/l α and EF-G/2, and catalytic and noncatalytic subunits of ATPase (ATPsynthase), archaebacteria are more closely related to eukaryotes than eubacteria. While this relatedness is certified, phylogenetic relationship among several major groups of archaebacteria (extreme thermophiles, extreme halophiles, and methanogens) and eukaryotes is still unsettled because the phylOgenetic tree topologies among them vary with the genes analysed. The ambiguous situation suggests that archaebacterial major groups and eukaryotes were diverged during very short period in the long course of evolution.