Some implications of a constant fiber stress hypothesis in the diastolic left ventricle

A thick-wall incompressible, elastic sphere was used as a model for the diastolic rat left ventricle. A model for myocardial nonhomogeneity was derived assuming that fiber (circumferential) stress was independent of position in the ventricular wall. The theoretical implications of the resulting constitutive relations together with the spherical model were analyzed in the context of large deformation elasticity theory. It was found that muscle stiffness at a given level of uniaxial stress increased monotonically from the endocardium to the epicardium. In addition, fiber stress was found to be essentially a linear function of transmural pressure above a pressure of 6 g/cm². It was also shown theoretically that neglecting the nonhomogeneity of the myocardium resulted in a state of stress which differed significantly from that predicted by the nonhomogeneous model. For example, at a transmural pressure of 14 g/cm², fiber stress in the nonhomogenous model was equal to 17 g/cm² while fiber stress in the homogeneous model varied between 100 g/cm² at the endocardial surface and 2 g/cm² at the epicardial surface. The change in muscle stiffness with position which characterized the nonhomogeneous model also tended to linearize the highly curvilinear radial stress distribution predicted by the homogeneous model at a given transmural pressure.     

The nuclear import of protein kinase D3 requires its catalytic activity

The protein kinase D (PKD) family consists of three serine/threonine protein kinases termed PKD, PKD2, and PKD3, which are similar in overall structure and primary amino acid sequence. However, each isozyme displays a distinctive intracellular localization. Taking advantage of the structural homology and opposite nuclear localization of PKD2 and PKD3, we generated an extensive set of chimeric proteins between both isozymes to determine which PKD3 domain(s) mediates its nuclear localization. We found that the C-terminal region of PKD3, which contains its catalytic domain, is necessary but not sufficient for its nuclear localization. Real time imaging of a photoactivatable green fluorescent protein fused to PKD3 revealed that point mutations that render PKD3 catalytically inactive completely prevented its nuclear import despite its interaction with importin alpha and beta. We also found that activation loop phosphorylation of PKD3 did not require its nuclear localization, and it was not sufficient to promote the nuclear import of PKD3. These results identify a novel function for the kinase activity of PKD3 in promoting its nuclear entry and suggest that the catalytic activity of PKD3 may regulate its nuclear import through autophosphorylation and/or interaction with another protein(s).     

Investigation of the occurrence of xylan-xyloglucan complexes in the cell walls of olive pulp (Olea europaea)

Previously we reported the possible occurrence of a complex containing glucuronoxylan and xyloglucan in the cell walls of olive pulp. In order to investigate the nature of this complex, the 1 M KOH (1 °C)-soluble polysaccharides in which it was prevalent, were separated by graded ethanol precipitation followed by anionexchange chromatography. A slightly acidic fraction was obtained and, by methylation analysis, glycosidic linkages typical of both xylan and xyloglucan were detected. Two distinct populations of the xylan-xyloglucan complexes were resolved by gel-filtration chromatography (2000 and 100 kDa) and the structural features were determined by methylation analysis. Cross-linking of the xylan-xyloglucan moieties was investigated by digestion of the xylan component with a purified, specific, endoxylanase. Although only the xylan element was digested, as verified by methylation analysis, the molecular weight of the xyloglucan moiety was also reduced. This confirmed that the xylan and xyloglucan moieties were strongly attached. The occurrence and structure of the xylan-xyloglucan complexes in the olive pulp cell walls is discussed.