The beta subunit of E. coli glycyl-tRNA synthetase plays a major role in tRNA recognition.

The contributions made by the alpha and beta subunits of E. coli glycyl-tRNA synthetase to the recognition of tRNA have been investigated via binding and immunological methods. Using the nitrocellulose filter assay, we have shown that isolated beta subunit, but not the alpha subunit, binds [14C]glycyl-tRNA with an affinity comparable to that of the native enzyme. Further, the data indicate that the beta subunit possesses one binding site for glycyl-tRNA while the native or reconstituted enzyme (alpha 2 beta 2) has two sites. Rabbit antibodies directed at the beta subunit or the holoenzyme inhibit efficiently the ability of the enzyme to aminoacylate tRNA while alpha-subunit antibodies have a smaller effect. Since none of the antisera have an appreciable effect on the ATP-PPi exchange activity of the enzyme under these conditions, the beta-subunit (and holoenzyme) antisera evidently interfere with productive tRNA binding. Taken together, the data indicate that the larger, beta subunit of glycyl-tRNA synthetase plays a major role in tRNA recognition.     

Organization of microtubules in stabilized meristematic plant cells revealed by a rat monoclonal antibody reacting only with the tyrosinated form of alpha-tubulin

A rat monoclonal antibody against yeast tubulin (clone YL 1/2; Kilmartin et al., 1982) that reacts specifically with mammalian alpha-tubulin carrying a carboxyterminal tyrosine residue (Wehland et al., 1983) was used to localize microtubules in plant cells derived from onion root apices (Allium cepa L.). YL 1/2 reacted with all types of microtubular arrays known to occur in higher plant meristematic cells such as interphase cortical microtubules, pre-prophase bands, the mitotic spindle and phragmoplast microtubules. The specific labeling of microtubules in isolated cells from onion root tips by YL 1/2 indicates that plant cells like animal cells contain tubulin tyrosine ligase, the enzyme which posttranslationally modifies alpha-tubulin. This enzyme could be involved in the dynamic regulation of microtubular arrays in all eukaryotic cells.     

Evidence for lysosomal enzyme recognition by human fibroblasts via a phosphorylated carbohydrate moiety

Adsorptive endocytosis of five different lysosomal enzymes from various human and non-human sources was susceptible to inhibition by mannose and l-fucose, methyl α-d-mannoside, α-anomeric p-nitrophenyl glycosides of mannose and l-fucose, mannose 6-phosphate and fructose 1-phosphate. A few exceptions from this general scheme were observed for particular enzymes, particularly for β-glucuronidase from human urine. The inhibition of α-N-acetylglucosaminidase endocytosis by mannose, p-nitrophenyl α-d-mannoside and mannose 6-phosphate was shown to be competitive. The loss of endocytosis after alkaline phosphatase treatment of lysosomal enzymes supports the hypothesis that the phosphorylated sugars compete with a phosphorylated carbohydrate on the enzymes for binding to the cell-surface receptors [Kaplan, Achord & Sly (1977) Proc. Natl. Acad. Sci. U.S.A. 74, 2026–2030]. Endocytosis of `low-uptake' forms of α-N-acetylglucosaminidase and α-mannosidase was likewise susceptible to inhibition by sugar phosphates and by alkaline phosphatase treatment, suggesting that `low-uptake' forms are either contaminated with `high-uptake' forms or are internalized via the same route as `high-uptake' forms. The existence of an alternative route for adsorptive endocytosis of lysosomal enzymes is indicated by the unaffected adsorptive endocytosis of rat liver β-glucuronidase in the presence of phosphorylated sugars and after treatment with alkaline phosphatase.     

Rod-like elements from actin-containing microfilament bundles observed in cultured cells after treatment with cytochalasin A (CA)

Immunofluorescence microscopy using antibody against actin has been used to study the expression of microfilamentous material in cells of a cloned mouse 3T3 line during cytochalasin A (CA) induced cell contraction. A conspicuous modification of the structure of the microfilament bundles is observed. Actin containing rod-like elements can be visualized both by phase contrast and immunofluorescence microscopy. These actin containing rods are of rather defined length (approximate length 5 μm) and seem to be derived as subunits from the original microfilament bundles. In some cells the rods were in the same orientation as the microfilament bundles in control cells, whereas other cells showed scattered arrangements. The phenomenon suggests intrafibrillar periodical heterogeneity in the microfilament bundles.     

Inhibition of protein synthesis by Cl-

Optimum K+ concentration for protein synthesis in four eukaryotic cell-free systems is obtained with 70 to 80 mM added KCl or with 110 to 150 mM added K(OAc). The different K+ optima are due to inhibition of protein synthesis by Cl- at concentrations higher than those present in the cytoplasm of eukaryotic cells. Initiation of protein synthesis is severely inhibited with 150 mM added KCl. This inhibition results from an impairment of mRNA binding to ribosomes. The binding of initiator Met-tRNAt, however, is only slightly inhibited by 150 mM KCl.     

Complexes of Escherichia coli lac-repressor with non-operator DNA revealed by electron microscopy: two repressor molecules can share the same segment of DNA.

Complexes of Escherichia coli lac-repressor with non-operator DNA have been visualized in the electron microscope using high-resolution metal shadowing and negative staining. Under conditions of a high ratio of repressor to DNA, all the DNA molecules are covered by repressor molecules and the resulting complexes appear as flattened ribbons with a width of approximately 200 Å. The overall dimensions of these complexes and their substructure indicate that it is very likely that repressor molecules are tightly packed on both “sides” of the DNA helix. Thus two repressor molecules can share the same segment of non-operator DNA by binding to opposite sides of the DNA helix.     

Resolution of optical isomers by high-pressure liquid chromatography: The separation of benzo[a]pyrene trans-diol derivatives

The optical isomers of (±)r-7,t-8-dihydroxy-7,8-dihydrobenzo[a]pyrene and its synthetic precursor (±)r-7,t-8-dihydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene were resolved as their di-(−)menthoxyacetates using high-pressure liquid chromatography. Saponification of the resolved diesters yielded the corresponding enantiomers. The specific rotation, CD spectra, and ORD curves are reported. The resolution of these optical isomers permits detailed studies on the enzymatic intermediates and the mechanism of benzo[a]pyrene activation to its carcinogenic form. The method is of general usefulness for the resolution of optical isomers.