Expression of a muscle-type alpha-actinin cDNA clone in non-muscle cells

We have previously isolated a chick smooth muscle-type alpha-actinin cDNA clone (C17) from a chick embryo fibroblast cDNA library. As part of an investigation into a possible role for a muscle isoform of alpha-actinin in non-muscle cells, we have cloned C17 into a eucaryotic expression vector, pKCR3, and examined the distribution of the expressed protein in non-muscle, monkey COS cells. We report here that the muscle isoform of chick alpha-actinin encoded by C17, was found in focal contacts and periodically distributed along actin filaments.     

Exogenous gangliosides enhance the interaction of fibronectin with ganglioside-deficient cells

The major cell-surface glycoprotein fibronectin mediates a variety of cellular adhesive interactions that have been reported to be competitively inhibited by gangliosides. These effects suggest a possible function of gangliosides as receptors for fibronectin. To test this hypothesis more directly, we examined the interaction of endogenous fibronectin with a ganglioside-deficient cell line, NCTC 2071. These cells, which grow in serum-free medium, synthesized fibronectin. The fibronectin did not bind to these cells, but instead bound diffusely to the culture substratum. When the cells were cultured in medium containing ganglioside, the fibronectin became bound to the cell surface in fibrillar strands. The order of effectiveness of purified gangliosides was GT1b greater than GD1a greater than GM1 greater than GM2 greater than GM3. The effect with mixed gangliosides was accompanied by a restoration of cellular capacity to bind and to respond to cholera toxin. Treatment of the cells with several phospholipids did not alter fibronectin binding. Our results support the hypothesis that gangliosides can help mediate the binding of fibronectin to fibroblasts.     

Synthesis of early heat shock proteins in young leaves of barley and sorghum

The in vivo synthesis of early heat-shock proteins in young leaves of barley (Hordeum vulgare L.) and sorghum (Sorghum bicolor L.) was studied by one- and two-dimensional electrophoresis. Analysis of whole leaf protein patterns demonstrated clearly the enhanced resolution of heat-shock proteins, especially those of low molecular weight, when separated by two-dimensional electrophoresis. Comparison between the two cereals showed that a greater number and diversity of heat-shock proteins were induced in the subtropical C₄ (sorghum) species compared to the temperate C₃ (barley) species. Fractionation of whole leaf proteins into soluble and membrane fractions showed the majority of heat-shock proteins to be associated with the soluble fraction in both sorghum and barley. However, several low molecular mass (17-24 kilodalton) heat-shock proteins were clearly identified in the membrane fractions, indicating a likely association with thylakoid membranes in vivo during the early stages of a heat-shock response in both species.     

Studies on the Mechanism of Photoinhibition in Higher Plants: I. EFFECTS OF HIGH LIGHT INTENSITY ON CHLOROPLAST ACTIVITIES IN CUCUMBER ADAPTED TO LOW LIGHT

Cucumber plants (Cucumis sativus L.), grown at low quantum flux density (120-150 microeinsteins per square meter per second) were photoinhibited by a three-hour exposure in air to ten times the light intensity experienced during growth. Chloroplasts were isolated from photoinhibited and control leaves and the following activities determined: O₂ evolution in the presence of ferricyanide, photosystem I activity, noncyclic and cyclic photophosphorylation, and light-induced proton uptake. Chlorophyll and chloroplast absorbance spectra, and chloroplast fluorescence were also measured. It was found that photosystem II electron transport and non-cyclic photophosphorylation were inhibited by about 50%, while cyclic photophosphorylation was less inhibited and photosystem I electron transport and light-induced proton uptake were unaffected. Electron transport to methylviologen could not be fully restored by electron donation to photosystem II. Chloroplast fluorescence induction at room temperature was strongly reduced following photoinhibition. There was no difference in the absorption spectra of the extracted chlorophylls from control and photoinhibited chloroplasts, but an increase of the absorption in the blue wavelength region was observed in the photoinhibited chloroplasts. It is suggested that high light stress does not result in alteration of the membrane properties, as is the case in low-temperature stress for example, but affects directly the photosynthetic reaction centers, primarily of photosystem II.