Sister chromatids are tethered together by the cohesin complex from the time they are made until their separation at anaphase. The ability of cohesin to tether sister chromatids together depends on acetylation of its Smc3 subunit by members of the Eco1 family of cohesin acetyltransferases. Vertebrates express two orthologs of Eco1, called Esco1 and Esco2, both of which are capable of modifying Smc3, but their relative contributions to sister chromatid cohesion are unknown. We therefore set out to determine the precise contributions of Esco1 and Esco2 to cohesion in vertebrate cells. Here we show that cohesion establishment is critically dependent upon Esco2. Although most Smc3 acetylation is Esco1 dependent, inactivation of the gene has little effect on mitotic cohesion. The unique ability of Esco2 to promote cohesion is mediated by sequences in the N terminus of the protein. We propose that Esco1-dependent modification of Smc3 regulates almost exclusively the noncohesive activities of cohesin, such as DNA repair, transcriptional control, chromosome loop formation, and/or stabilization. Collectively, our data indicate that Esco1 and Esco2 contribute to distinct and separable activities of cohesin in vertebrate cells.

Background: HIV-1 Nef targets the coreceptor CD4 to the multivesicular body (MVB) pathway for degradation via an unknown mechanism. Results: Nef interacts with Alix in late endosomes, and this is required for efficient lysosomal targeting of CD4. Conclusion: Nef utilizes Alix as an adaptor to target CD4 for lysosomal degradation. Significance: The study clarifies the mechanism by which Nef down-regulates expression of specific host-cell proteins.

The HIV accessory protein Nef is a major determinant of viral pathogenesis that facilitates viral particle release, prevents viral antigen presentation and increases infectivity of new virus particles. These functions of Nef involve its ability to remove specific host proteins from the surface of infected cells, including the CD4 receptor. Nef binds to the adaptor protein 2 (AP-2) and CD4 in clathrin-coated pits, forcing CD4 internalization and its subsequent targeting to lysosomes. Herein, we report that this lysosomal targeting requires a variant of AP-1 containing isoform 2 of γ-adaptin (AP1G2, hereafter γ2). Depletion of the γ2 or μ1A (AP1M1) subunits of AP-1, but not of γ1 (AP1G1), precludes Nef-mediated lysosomal degradation of CD4. In γ2-depleted cells, CD4 internalized by Nef accumulates in early endosomes and this alleviates CD4 removal from the cell surface. Depletion of γ2 also hinders EGFR-EGF-complex targeting to lysosomes, an effect that is not observed upon γ1 depletion. Taken together, our data provide evidence that the presence of γ1 or γ2 subunits delineates two distinct variants of AP-1 complexes, with different functions in protein sorting.

Chromosome structure in both interphase and M phase cells is strongly influenced by the action of the Cohesin and Condensin protein complexes. The Cohesin complex tethers the identical copies of each chromosome, called sister chromatids, together following DNA replication, and promotes normal interphase chromosome structure and gene expression. In contrast, Condensin is active largely in M phase and promotes the compaction of individual chromosomes. The Xenopus egg extract system provides a uniquely suitable system with which to analyze the functions of both Cohesin and Condensin. Egg extracts, in which the cell cycle state can be manipulated, contain stockpiles of nuclear proteins, including Condensin and Cohesin, sufficient for the assembly of thousands of nuclei per microliter. Egg extract prepared from unfertilized eggs is arrested by the presence of cytostatic factor (CSF) in a state with high levels of M-phase kinase activity, but can be stimulated to enter interphase, in which DNA replication occurs spontaneously. For cohesion assays, demembranated sperm nuclei are incubated in interphase extract, where they undergo rapid and synchronous DNA replication and cohesion establishment through the recruitment of proteins and other factors (e.g. nucleotides) from the extract. Sister chromatid cohesion is assessed by then driving the extract into M phase by the addition of fresh CSF-arrested extract. Chromosome condensation occurs spontaneously in M phase extract extracts. Sperm nuclei are therefore added directly to CSF extracts to assay condensation. In the following protocols, we describe basic assays for Cohesin and Condensin function using Xenopus egg extracts (see Figure 1 for schematic overview). Materials Reagents All water for solution preparation is ultrapure (~18.2 MM•cm at 25 °C). Adenosine triphosphate, disodium salt (ATP) (200 mM in H 2 O).