Hysteresis and conformational drift of pressure-dissociated glyceraldehydephosphate dehydrogenase

Pressure dissociation of yeast glyceraldehydephosphate dehydrogenase (GAPDH) was studied by fluorescence spectroscopy. Observations in the range of -5 to 30 degrees C indicate that monomer association into the tetramer proceeds with an enthalpy change of -14 kcal mol-1 and a large increase in entropy which at 25 degrees C amounts to 18 kcal mol-1. The large conformational drift and the low-temperature stability of the tetramer recovered after decompression facilitated a comparison of its properties with those of the native tetramer. Significant differences in absorption and fluorescence-excitation polarization spectra, yield of tryptophan fluorescence, and binding of anilinonaphthalenesulfonate and NADH were observed. At 0 degree C the standard free energies of association of the monomers into the native and drifted tetramers were respectively -32 and -29 kcal mol-1. The volume change upon association measured from the pressure span of the compression curves was 200-230 mL mol-1 but four times as large when derived from the displacement of the compression curves with total protein concentration. This large discrepancy can be explained by the existence in the native tetramer population of a distribution of free energies of association with a dispersion from the mean of about 6 kcal mol-1. At 0 degree C and 1 bar ATP and ADP decreased the stability of the GAPDH tetramer by changes in free energy of association of +3.7 and +4.1 kcal mol-1, respectively. NAD and c-AMP stabilized it by -2.3 and -1.3 kcal mol-1. The variation in sign and magnitude of the ligand-induced changes in free energy of association observed in this case, and previously in hexokinase [Ruan, K., & Weber, G. (1988) Biochemistry 27, 3295], and the heterogeneity of the free energy of association of GAPDH, revealed as indicated above, lead to the conclusion that oligomeric aggregates exist in a variety of conformations that depend upon the protein concentration, temperature, pressure, and the presence of specific ligands. The multiplicity of species revealed by the energetics raises questions about the significance of the structures of oligomeric proteins determined by X-ray crystallography.     

Inhibition of epidermal growth factor receptor biosynthesis caused by the src oncogene product, pp60v-src.

We have previously shown that an intracellular mechanism down regulates epidermal growth factor (EGF) receptor levels in rodent fibroblasts transformed by the src oncogene (W. J. Wasilenko, L. K. Shawver, and M. J. Weber, J. Cell. Physiol. 131:450-457, 1987). We now report that this down regulation is due to an inhibition of EGF receptor biosynthesis. With Rat-1 (R1) cells infected with a temperature-sensitive src mutant, we found that 125I-labeled EGF binding to cells began to decrease soon after the activation of pp60v-src by shift down to the permissive temperature for transformation. This effect of src on EGF receptors was reversible. Pulse-chase studies with [35S]methionine-labeled cells revealed that the tyrosine protein kinase activity of pp60v-src had little if any effect on EGF receptor degradation rate. By contrast, the expression of pp60v-src caused a large reduction in the apparent rate of EGF receptor biosynthesis. Northern (RNA) blot analysis demonstrated that pp60v-src also caused marked reductions in the steady-state level of EGF receptor mRNA. These data indicate that one way the expression of the src oncogene can affect the machinery of growth control is by affecting the expression of specific genes for growth factor receptors.     

SEM observations on seeds ofStriga spp. andBuchnera americana (Scrophulariaceae)

Seeds of the root parasitesStriga (several spp.) andBuchnera americana were examined by means of SEM. The surface patterns of the seeds in both genera resemble each other closely, especially those ofS. angustifolia andB. americana. SomeStriga spp. can be clearly distinguished by their surface characteristics, while this is quite difficult in others. The taxonomic value of the seed surface features ofStriga andBuchnera is discussed.