Packing forces in ribonuclease crystals

Packing in Ribonuclease A and Ribonuclease S crystals have been compared in order to determine the possible role of the precipitant on lattice contacts. Both proteins have similar tertiary structures, but they crystallize in different space groups depending on the precipitating agent. It is found that packing differs either by the number of nearest neighbours or by the size of surface areas buried in individual contacts. Ammonium sulfate seems to promote hydrophobic interactions with interfaces similar to those found in oligomeric proteins. Organic precipitants favour electrostatic interactions.     

Directional fixation of mutations in vertebrate evolution

Summary We have made pairwise comparisons between the coding sequences of 21 genes from coldblooded vertebrates and 41 homologous sequences from warm-blooded vertebrates. In the case of 12 genes, GC levels were higher, especially in third codon positions, in warm-blooded vertebrates compared to cold-blooded vertebrates. Six genes showed no remarkable difference in GC level and three showed a lower level. In the first case, higher GC levels appear to be due to a directional fixation of mutations, presumably under the influence of body temperature (see Bernardi and Bernardi 1986b). These GC-richer genes of warm-blooded vertebrates were located, in all cases studied, in isochores higher in GC than those comprising the homologous genes of cold-blooded vertebrates. In the third case, increases appear to be due to a limited formation of GC-rich isochores which took place in some cold-blooded vertebrates after the divergence of warm-blooded vertebrates. The directional changes in the GC content of coding sequences and the evolutionary conservation of both increased and unchanged GC levels are in keeping with the existence of compositional constraints on the genome.     

Tyrosine protein kinase activity of the EGF receptor is required to induce activation of receptor-operated calcium channels

We have investigated the effects of epidermal growth factor (EGF) on calcium ion channels in A431 epidermoid carcinoma cells. We have found that: -1- EGF stimulates Ca2+ channels. -2- EGF stimulated Ca2+ channels are voltage independent, exhibit a low conductance (8 pS) and a bursting multichannels activity (BMC). -3- Activation of the tyrosine-kinase function of the EGF receptor is required to generate Ca2+ current. -4- Inositol (1,4,5) triphosphate (Ins (1,4,5) P3) and EGF have similar effect on the channel activation. These results suggest that: stimulation of tyrosine-kinase activity of the EGF receptor, production of Ins (1,4,5)P3 and calcium entry via voltage independent channels are important connected steps in mediating the mitogenic effect of this growth factor.     

Evidence for a novel growth factor in Xenopus oocytes

Xenopus oocytes contain a novel growth factor, as determined by its effect on the anchorage-dependent and -independent growth of various rat cells and on a Xenopus cell line; it is destroyed by trypsin, acidification (pH 2.0), heating (100 degrees C, 3 min), 8 M urea but not by dithiothreitol. Gel filtration of this activity in nondissociating conditions suggests the existence of aggregates and the presence of a very high (approximately 2000 Kd) and a much lower (approximately 30 Kd) form.     

Nucleotide sequence of Acinetobacter baumannii aphA-6 gene: evolutionary and functional implications of sequence homologies with nucleotide-binding proteins, kinases and other aminoglycoside-modifying enzymes

A new kanamycin-resistance gene, detected in Acinetobacter baumannii and designated aphA-6, was sequenced. It specifies a 30319 Dalton 3'-aminoglycoside phosphotransferase (APH(3'] that mediates resistance to kanamycin and structurally related aminoglycosides, including amikacin. Pairwise comparisons of the six types of APH(3') so far detected in human pathogens (types I, II, III and VI) and in amino-glycoside-producing microorganisms (types IV and V), confirm that APH(3') enzymes have diverged from a common ancestor. Three highly retained motifs (1: V--HGD----N; 2: G--D-GR/K-G and 3: D--K/R--Y/F---LDE) located in the C-terminal part of the enzymes were defined. Screening of protein sequence data bases fore each of these motifs revealed that motifs 1 and 2 are both found in nucleotide-binding phosphotransferases associated with a variety of biological processes, namely adenylate kinase, viral oncogenic protein kinases, elongation factors, Na+/K+-transporting ATPase, myosin and antibiotic-modifying enzymes. Motif 2 probably corresponds to the MgATP binding site, while motifs 3 and 1 could be involved in the splitting of the phosphodiester bond and in the phosphate transfer, respectively. Moreover, an additional motif, almost invariably centrally located, was found in all aminoglycoside-modifying enzymes. The occurrence of this motif, possibly a recombination site which would have allowed the association of units of separate functions, is compatible with a modular concept for the structure of aminoglycoside-modifying enzymes.     

Anchorage-dependent surface distribution and partition during freeze-fracture of viral transmembrane glycoproteins

We have compared in the same cell type the surface distribution and partition in freeze-fractured plasma membranes of Sindbis virus glycoproteins in three different situations: (i) in permanently transformed cells that express the glycoproteins as the only viral product; (ii) in cells in which prebound viruses were forced to fuse with the plasma membrane by low pH treatment; (iii) in virus-infected cells. We report here that the viral proteins expressed on the surface of transfected cells show a uniform and unclustered distribution; conversely, in Sindbis virus-infected cells they appear clustered, regionally distributed, and always associated with budding viruses (i.e., interacting with the nucleocapsid on the cytosolic side of the membrane). Furthermore, the viral proteins expressed on transfected cells or implanted by low pH-mediated fusion partition during freeze-fracture with the exoplasmic faces of the cell plasma membranes, whereas an opposite partition is observed in infected cells. These results strongly suggest that in infected cells the clustering and the partition with the protoplasmic faces of the plasma membrane depend only on the strong "anchorage" of the glycoproteins to the nucleocapsid.