Free and polymerized tubulin in cultured bone cells and Chinese hamster ovary cells: the influence of cold and hormones

A low pH method of liposome-membrane fusion (Schneider et al., 1980, Proc. Natl. Acad. Sci. U. S. A. 77:442) was used to enrich the mitochondrial inner membrane lipid bilayer 30-700% with exogenous phospholipid and cholesterol. By varying the phospholipid-to- cholesterol ratio of the liposomes it was possible to incorporate specific amounts of cholesterol (up to 44 mol %) into the inner membrane bilayer in a controlled fashion. The membrane surface area increased proportionally to the increase in total membrane bilayer lipid. Inner membrane enriched with phospholipid only, or with phospholipid plus cholesterol up to 20 mol %, showed randomly distributed intramembrane particles (integral proteins) in the membrane plane, and the average distance between intramembrane particles increased proportionally to the amount of newly incorporated lipid. Membranes containing between 20 and 27 mol % cholesterol exhibited small clusters of intramembrane particles while cholesterol contents above 27 mol % resulted in larger aggregations of intramembrane particles. In phospholipid-enriched membranes with randomly dispersed intramembrane particles, electron transfer activities from NADH- and succinate-dehydrogenase to cytochrome c decreased proportionally to the increase in distance between the particles. In contrast, these electron- transfer activities increased with decreasing distances between intramembrane particles brought about by cholesterol incorporation. These results indicate that (a) catalytically interacting redox components in the mitochondrial inner membrane such as the dehydrogenase complexes, ubiquinone, and heme proteins are independent, laterally diffusible components; (b) the average distance between these redox components is effected by the available surface area of the membrane lipid bilayer; and (c) the distance over which redox components diffuse before collision and electron transfer mediates the rate of such transfer.     

Crystallography of liver alcohol dehydrogenase complexed with substrates.

The cytoplasmic metabolism of two adenovirus (Ad2)-specific messenger RNAs, encoded within the left-hand 11% of the Ad2 genome, has been examined in a transformed rat embryo line (strain 8617). Both the accumulation of labeled mRNA and the decay of pulse-labeled mRNA after an actinomycin D chase indicate a stochastic turnover of these Ad2-specific mRNA with half lives of approximately 35 and 100 minutes, considerably shorter than the half-life of the bulk of mRNA.The separation of mRNA molecules according to the length of poly(A) showed that the poly(A) of the Ad2-specific mRNAs was shortened both during continuous and pulse-chase labeling. Moreover, consistent with the observed rates of mRNA turnover, the rate of poly(A) shortening in the Ad2 mRNAs was found to be faster than that of the bulk mRNA population. In addition, the results suggested that both mRNA turnover and poly(A) shortening proceeded by random endonucleolytic cleavage.