ABSTRACT
Recently we developed a method called direct interaction rescue (DIRE) for selective cloning in filamentous phage. The rescue is effected by the interaction of two heterologous proteins, one fused to the N-terminus of gene 3 adhesion protein, the other fused to the C-terminus. When heterologous fusion proteins interact with each other, gene 3 protein activity in restored thereby rescuing phage infectivity. We have used the leucine zipper of c-Jun protein as a 'bait' to select for interacting proteins from a human cDNA library. Two interacting clones were isolated, one coding for ribosomal protein L18a, a component of the large ribosomal subunit, and the other for tropomyosin, a component of the cytoskeleton. L18a contains two zipper-like domains which probably interact with c-Jun. We consider it possible that L18a (and tropomyosin) are involved in the cellular regulation of Jun protein levels.
Subject(s)
Gene Expression Regulation/physiology , Genes, jun , Leucine Zippers/genetics , Proto-Oncogene Proteins c-jun/biosynthesis , Ribosomal Proteins/metabolism , Amino Acid Sequence , Bacteriophages/genetics , Bacteriophages/metabolism , Base Sequence , Humans , Molecular Sequence Data , Oncogene Proteins, Fusion/genetics , Oncogene Proteins, Fusion/metabolism , Proto-Oncogene Proteins c-jun/genetics , Tropomyosin/genetics , Tropomyosin/metabolismABSTRACT
HIV-1 reverse transcriptase is a dimeric enzyme mainly involved in the replication of the viral genome. A filamentous phage cDNA expression library from human lymphocytes was used to select cellular proteins interacting with HIV-1 reverse transcriptase Affinity selections using the bacterially expressed monomeric large subunit of reverse transcriptase (p66) yielded host beta-actin. This clone was expressed as glutathione-S-transferase fusion protein which was identified by using a specific antibody against beta-actin. Furthermore we show that also the eukaryotic beta-actin binds to either the large subunit of reverse transcriptase or to the Pol precursor polyprotein in vitro. The reverse transcriptase/beta-actin interaction might be important for the secretion of HIV-1 virions.
Subject(s)
Actins/metabolism , HIV-1/enzymology , RNA-Directed DNA Polymerase/metabolism , Animals , Base Sequence , Cattle , DNA Primers , DNA, Complementary , Gene Products, gag/metabolism , Glutathione Transferase/metabolism , HIV Reverse Transcriptase , Humans , Molecular Sequence Data , Protein Binding , Protein Precursors/metabolism , Protein Processing, Post-Translational , Recombinant Fusion Proteins/metabolism , gag Gene Products, Human Immunodeficiency Virus , pol Gene Products, Human Immunodeficiency VirusABSTRACT
A prokaryotic CpG-specific methylase from Spiroplasma, SssI methylase, is now widely used to study the effect of CpG methylation in mammalian cells, and can processively modify cytosines in CpG dinucleotides in the absence of Mg2+. In the presence of Mg2+, we found (i) that the methylation reaction is distributive rather than processive as a result of the decreased affinity of SssI methylase for DNA, and (ii) that a type I-like topoisomerase activity is present in SssI methylase preparations. This topoisomerase activity was still present in SssI methylase further purified by either SDS-polyacrylamide or isoelectric focusing gel electrophoresis. We show that methylase and topoisomerase activities are not functionally interdependent, since conditions exist where only one or the other enzymatic activity is detectable. The catalytic domains of SssI methylase and prokaryotic topoisomerases show similarity at the amino acid level, further supporting the idea that the topoisomerase activity is a genuine activity of SssI methylase. Mycoplasmas, including Spiroplasma, have the smallest genomes of all living organisms; thus, this condensation of two enzymatic activities into the same protein may be a result of genome economy, and may also have functional implications for the mechanism of methylation.
