CYLD: a DUB with many talents

The deubiquitinating enzyme CYLD is a tumor suppressor protein known for its role in repression of generally pro-oncogenic NF-kappaB activation pathways. Two new studies published in this and the September issue of Developmental Cell show that CYLD dismantles distinct types of polyubiquitin chains formed on select signaling proteins and is thereby required for normal vertebrate and invertebrate development.     

Endocytosis: Actin in the driving seat

Endocytosis is an essential eukaryotic process which has been reported to require a functional actin cytoskeleton. New data obtained using the model yeast Saccharomyces cerevisiae significantly advances our understanding of the interface between the endocytic machinery and actin.     

Endocytosis: driving membranes around the bend

When a nascent vesicle buds, the membrane must curve. Several mechanisms have been proposed for curvature creation or stabilization. Structural analysis of the ENTH domain of the endocytic protein epsin has suggested a new mechanism, in which the ENTH domain pushes its way into membranes, thus bending them into shape.     

Endocytosis: curvature to the ENTH degree

Recent work has shown that the protein epsin 1 induces highly curved lipidic structures when added with clathrin to appropriate lipid mixtures. This property may be a critical factor in the 'curvature stress cycle' of membrane trafficking.     

Endocytosis: clathrin-mediated membrane budding

Clathrin-dependent endocytosis is the major pathway for the uptake of nutrients and signaling molecules in higher eukaryotic cells. The long-held tenet that clathrin-coated vesicles are created from flat coated plasma membrane patches by a sequential process of invagination, bud formation and fission recently received strong support from the results of advanced live cell fluorescence microscopy. The data on the critical components that deform the plasma membrane locally into a coated bud suggest that membrane bending is a team effort requiring membrane-curving protein domains, actin dynamics and, last but not least, clathrin. The scission step requires the mechano-enzymatic function of dynamin, actin dynamics and possibly myosin motor proteins. Finally, a burst of auxilin/GAK initiates the uncoating of the vesicle.     

Catching a DUB in the act: novel ubiquitin-based active site directed probes

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Regulation of monoubiquitinated PCNA by DUB autocleavage

Monoubiquitination is a reversible post-translational protein modification that has an important regulatory function in many biological processes, including DNA repair. Deubiquitinating enzymes (DUBs) are proteases that are negative regulators of monoubiquitination, but little is known about their regulation and contribution to the control of conjugated-substrate levels. Here, we show that the DUB ubiquitin specific protease 1 (USP1) deubiquitinates the DNA replication processivity factor, PCNA, as a safeguard against error-prone translesion synthesis (TLS) of DNA. Ultraviolet (UV) irradiation inactivates USP1 through an autocleavage event, thus enabling monoubiquitinated PCNA to accumulate and to activate TLS. Significantly, the site of USP1 cleavage is immediately after a conserved internal ubiquitin-like diglycine (Gly-Gly) motif. This mechanism is reminiscent of the processing of precursors of ubiquitin and ubiquitin-like modifiers by DUBs. Our results define a regulatory mechanism for protein ubiquitination that involves the signal-induced degradation of an inhibitory DUB.     

Endocytosis of HIV: anything goes

The major pathway for HIV internalization in CD4+ T cells has been thought to be the direct fusion of virus and cell membranes, because the cell surface is the point of entry of infectious particles. However, the exact contribution of endocytic pathways to the infection of CD4+ T lymphocytes is unknown, and the mechanisms involved in endocytosis of HIV particles are unclear. Recent evidence suggests that endocytosis of cell-free and cell-associated virus particles could lead to effective virus entry and productive infections. Such observations have, in turn, spurred a debate on the relevance of endosomal entry as a mechanism of escape from the immune system and HIV entry inhibitors. In this paper, we review the endocytosis of HIV and discuss its role in HIV infection and pathogenesis.     

DeubiKuitylation: A novel DUB enzymatic activity for the DNA repair protein,Ku70

The Ku70 protein was shown to be involved in multiple cellular pathways including DNA repair, telomere maintenance, V(D)J recombination and Bax mediated apoptosis. Yet, despite this wide spectrum of pathways, till recently the enzymatic activity of Ku70 was elusive. Recent findings demonstrate that Ku70 is associated with the proapoptotic protein Bax and possesses a deubiquitin enzyme (DUB) activity on it. These data suggest a dual role for Ku70 in apoptotic regulation; on one hand the association with Ku70 sequestered Bax away from the mitochondria and plays an antiapoptotic function. On the other hand, this association mediates and promotes Bax deubiquitination which will block its labeling for proteasomal degradation and in this manner Ku70 will have a proapoptotic role. The exciting finding of Ku70’s DUB activity opens numerous avenues for future research. Here we suggest candidate substrate proteins and indicate how the DUB activity of Ku70 on these, might affect the known Ku70’s related pathways.     

Endocytosis: EH domains lend a hand

A number of proteins that have been implicated in endocytosis feature a conserved protein-interaction module known as an EH domain. The three-dimensional structure of an EH domain has recently been solved, and is likely to presage significant advances in understanding molecular mechanisms of endocytosis.     

Endocytosis of neurotransmitter receptors: location matters

Endocytosis of excitatory glutamate receptors from the postsynaptic plasma membrane plays a fundamental role in synaptic function and plasticity. In a recent study published in Neuron, Lu et al. (2007) describe protein interactions that link zones of receptor endocytosis directly to the postsynaptic scaffold and propose that local trafficking of receptors facilitated by these endocytic zones is required to maintain synaptic responsiveness.     

Endocytosis in plants

Endocytosis in animal cells has been heavily documented. Both fluid-phase and receptor-mediated modes of uptake have been frequently studied, and the endocytic pathway is well defined. This contrasts markedly with the situation in plants where our knowledge of this process is still rudimentary. Partly responsible for this situation has been the view, widely held among plant physiologists, that because of turgor, endocytosis cannot occur in plant cells. As discussed below, the case against endocytosis is no longer water-tight.
Endocytosis is a fact in protoplasts. It can be demonstrated with electron-dense tracers as well as with membrane impermeant dye Lucifer Yellow CH. The latter has also been used with success on both suspension-cultured and tissue cells of higher plants, suggesting that fluid-phase endocytosis is also a feature of cells with walls. Through the application of fluorescently labelled elicitor molecules, which specifically bind to the cell surface of suspension-cultured cells, it has also been possible to provide convincing evidence for the operation of receptor-mediated endocytosis in plants. A number of studies on protoplasts and cells clearly indicate that endocytosis in plants can be mediated by coated pits in the plasma membrane. At least one of the organelles that lie on the endocytic pathway in plants has a structurally similar counterpart in animal cells: the multivesicular body. The first recipient of internalized molecules is the partially coated reticulum, although its relationship to the Golgi apparatus and Golgi function remain to be clarified. The final target for endocytosis in plants appears to be the vacuole.