A series of extraction procedures were applied to avian nuclei which allowed us to define three types of association of v-myc-and c-myc-encoded proteins with nuclei: (i) a major fraction (60 to 90%) which is retained in DNA-and RNA-depleted nuclei after low-and high-salt extraction, (ii) a small fraction (1%) released during nuclease digestion of DNA in intact nuclei in the presence of low-salt buffer, and (iii) a fraction of myc protein (<10%) extractable with salt or detergents and found to have affinity for both single-and double-stranded DNA. Immunofluorescence analysis with anti-myc peptide sera on cells extracted sequentially with nucleases and salts confirmed the idea that myc proteins were associated with a complex residual nuclear structure (matrix-lamin fraction) which also contained avian nuclear lamin protein. Dispersal of myc proteins into the cytoplasm was found to occur during mitosis. Both c-myc and v-myc proteins were associated with the matrix-lamin, suggesting that the function of myc may relate to nuclear structural organization.Recent evidence indicates that the proteins encoded by retroviral oncogenes fall into two classes with respect to their predominant subcellular localizations. The proteins encoded by the rather diverse oncogene families showing various degrees of relatedness to src or ras have been known for some time to be present in the cytoplasmic or plasma membrane compartments or in both. Recently it has been shown that the protein products of the myc, myb, and fos genes have a nuclear localization (37). Although gross subcellular location provides only a hint as to oncogene function, the nuclear localization of myc proteins (1,4,16,23) is especially intriguing in light of evidence suggesting that myc may relate to cell growth control (10,28,32,38). In addition it appears that alterations at the myc locus may be underlying events in many lymphoid and at least several nonlymphoid neoplasms in birds, rodents, and humans. The role that myc plays in these events, however, is unknown.In principle, three classes of myc-related proteins can be considered: the v-myc-encoded proteins produced by the avian acute leukemia viruses; the c-myc protein synthesized after translocation, amplification, or promoter insertion in the vicinity of the c-myc allele; and the c-myc protein produced by an unaltered normal c-myc gene. In the avian system the group of acute leukemia viruses which possess the myc oncogene synthesize their v-myc proteins as either fusion products containing viral structural protein regions (usually derived from the gag gene which specifies the internal structural protein precursor) linked to v-myc protein regions (MC29 pjj0gag-mYc, CMII p9fgag-myc OK10 P200gag-pol-myc) or as v-myc proteins apparently not linked to any other viral proteins (OK10 p62v-mYc; MH2 p61/63vmYc) (4,23,26). In avian bursal lymphoma cell lines, in which c-myc transcription has been increased due to proximal integration of a retroviral long terminal repeat, a 62,000-dalton protein has been identified as t...
Integration of the viral genome into the nuclear DNA of a host cell plays a pivotal role in the replication of retroviruses. We have developed an in vitro method for studying the biochemistry of human immunodeficiency virus (HIV) integration by using extracts from HIV-infected cells. Analysis of the reaction products showed that HIV integration in vitro accurately reproduces the in vivo process. Integration occurred without apparent specificity for the target sequence, and the integrated provirus was directly flanked by a 5-base-pair duplication of DNA from the target site. HIV integration did not require a high-energy cofactor, and the enzymatic activities required for integration were recovered with the viral DNA when cell extracts were fractionated by gel exclusion chromatography.
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