The HIV-1 genome contains several genes coding for auxiliary proteins, including the small Vpr protein. Vpr affects the integrity of the nuclear envelope and participates in the nuclear translocation of the preintegration complex containing the viral DNA. Here, we show by photobleaching experiments performed on living cells expressing a Vpr-green fluorescent protein fusion that the protein shuttles between the nucleus and the cytoplasm, but a significant fraction is concentrated at the nuclear envelope, supporting the hypothesis that Vpr interacts with components of the nuclear pore complex. An interaction between HIV-1 Vpr and the human nucleoporin CG1 (hCG1) was revealed in the yeast twohybrid system, and then confirmed both in vitro and in transfected cells. This interaction does not involve the FG repeat domain of hCG1 but rather the N-terminal region of the protein. Using a nuclear import assay based on digitonin-permeabilized cells, we demonstrate that hCG1 participates in the docking of Vpr at the nuclear envelope. This association of Vpr with a component of the nuclear pore complex may contribute to the disruption of the nuclear envelope and to the nuclear import of the viral DNA.In contrast to oncoretroviruses that replicate only in dividing cells and enter the nucleus upon nuclear breakdown during mitosis, HIV-1 1 and other lentiviruses have the ability to infect non-dividing cells, such as macrophages and quiescent T lymphocytes. After entry of the virus into the cell, the HIV capsid seems to uncoat rapidly. The genomic HIV-1 RNA is reverse transcribed into linear double-stranded DNA, which remains associated with a nucleoprotein complex, called the preintegration complex (PIC). The viral DNA is then imported into the nucleus through the nuclear envelope (NE) via an active mechanism within 4 -6 h after infection (1).In eukaryotic cells, the NE creates distinct nuclear and cytoplasmic compartments. This structure consists of two concentric membranes, the inner and outer nuclear membranes which are continuous with the endoplasmic reticulum. The NE is stabilized by the nuclear lamina, a tight meshwork of intermediate filament proteins underlying the inner nuclear membrane (for review, see Ref.2), whereas on the outer side, cytoplasmic intermediate filaments in close contact with the nucleus serve to suspend the nucleus in the cytoplasm (3). Spanning both membranes are nuclear pore complexes (NPC) that form aqueous channels, which allow selective traffic between nucleus and cytoplasm and impose a permeability barrier to free diffusion of macromolecules or complexes. The NPC is a large supramolecular structure (4) formed of ϳ30 unique proteins in vertebrates, termed nucleoporins (Nups) giving rise to an estimated molecular mass of 60 MDa (Ref. 5; for reviews, see Refs. 6 and 7). High resolution electron microscopic images of NPCs reveal an 8-fold symmetric structure, formed by nuclear and cytoplasmic rings and a central spoke complex. Peripheral filaments emanate from the core of the complex into the nucle...

c-Myc is a predominately nuclear transcription factor that is a substrate for rapid turnover by the proteasome system. Cancer-related mutations in c-Myc lead to defects in its degradation and thereby contribute to the increase in its cellular level that is associated with the disease. Little is known about the mechanisms that target c-Myc to the proteasomes. By using a GFP fusion protein and live analysis we show that c-Myc shuttles between the nucleus and cytoplasm and thus it could be degraded in either compartment. Strikingly, at elevated levels of expression c-Myc accumulates at nucleoli in some cells,consistent with saturation of a nucleolus-associated degradation system in these cells. This idea is further supported by the observation that proteasome inhibitor treatment causes accumulation of c-Myc at the nucleoli of essentially all cells. Under these conditions c-Myc is relatively stably associated with the nucleolus, as would be expected if the nucleolus functions as a sequestration/degradation site for excess c-Myc. Furthermore, during elevated c-Myc expression or proteasome inhibition, nucleoli that are associated with c-Myc also accumulate proteasomes. c-Myc and proteasomes co-localise in intranucleolar regions distinct from the dense fibrillar component of the nucleolus. Based on these results we propose a model for c-Myc downregulation where c-Myc is sequestered at the nucleoli. Sequestration of c-Myc is accompanied by recruitment of proteasomes and may lead to subsequent degradation.

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Gp210, an integral membrane protein of the nuclear pore complex (NPC), is believed to be involved in NPC biogenesis. To test this hypothesis, we have investigated dynamic properties of the NPC and distribution of NPC proteins in NIH/ 3T3 cells lacking gp210. POM121 (the other integral NPC protein) and NUP107 (of the NUP107/160 complex) were correctly distributed at the nuclear pores in the absence of gp210. Furthermore, fluorescence recovery after photobleaching experiments showed that POM121 and NUP107 remained stably associated at the NPCs. We conclude that gp210 cannot be required for incorporation of POM121 or NUP107 or be required for maintaining NPC stability.