Neutron diffraction study of aqueous Laponite suspensions at the NIMROD diffractometer.

The process of dynamical arrest, leading to formation of different arrested states such as glasses and gels, along with the closely related process of aging, is central for both basic research and technology. Here we report on a study of the time-dependent structural evolution of two aqueous Laponite clay suspensions at different weight concentrations. Neutron diffraction experiments have been performed with the near and intermediate range order diffractometer (NIMROD) that allows studies of the structure of liquids and disordered materials over a continuous length scale ranging from 1 to 300 Å, i.e., from the atomistic to the mesoscopic scales. NIMROD is presently a unique diffractometer, bridging the length scales traditionally investigated by small angle neutron scattering or small angle x-ray scattering with that accessible by traditional diffractometers for liquids. Interestingly, we have unveiled a signature of aging of both suspensions in the length scale region of NIMROD. This phenomenon, ascribed to sporadic contacts between Laponite platelets at long times, has been observed with the sample arrested as gel or as repulsive glass. Moreover, water molecules within the layers closest to Laponite platelets surface show orientational and translational order, which maps into the crystalline structure of Laponite.

Differential localization to cytoplasm, nucleus or P-bodies of yeast PKA subunits under different growth conditions.

Our aim in this work was to further characterize the complexity and specificity of the three different isoforms (Tpk1, Tpk2 and Tpk3) of the catalytic and regulatory (Bcy1) subunits of PKA in Saccharomyces cerevisiae. We thus analyzed the subcellular localization of the PKA subunits in living cells by using strains carrying GFP (green fluorescent protein) fused to each PKA subunit. During exponential growth on glucose, both Bcy1 and Tpk2 localized in the nucleus, whereas Tpk1 and Tpk3 showed a mixed pattern of nucleo-cytoplasmic localization. During exponential growth on glycerol and during stationary phase, the PKA subunits showed mostly cytoplasmic localization, with the peculiarity that Tpk2 and Tpk3 but not Bcy1, were found associated to P-bodies and EGP bodies. Tpk3 was accumulated into P-bodies during glucose deprivation and hyper osmotic stress. Deletion of Tpk3 altered the kinetics of P-body formation. Analysis of protein expression showed that the relative expression pattern of each Tpk changes from low levels under fermentative metabolism to higher levels during stationary phase. The increase in Tpk levels produced an imbalance with Bcy1 levels. Our data suggest that the signaling specificity through PKA in yeast could be mediated by a particular subcellular localization of each isoform of Tpk.