CiteULike: lechristophe fret

Imaging the biogenesis of individual HIV-1 virions in live cells

Nolwenn Jouvenet — 2008-05-26T13:58:20-00:00

Nature (25 May 2008)

Observations of individual virions in live cells have led to the characterization of their attachment, entry and intracellular transport. However, the assembly of individual virions has never been observed in real time. Insights into this process have come primarily from biochemical analyses of populations of virions or from microscopic studies of fixed infected cells. Thus, some assembly properties, such as kinetics and location, are either unknown or controversial. Here we describe quantitatively the genesis of individual virions in real time, from initiation of assembly to budding and release. We studied fluorescently tagged derivatives of Gag, the major structural component of HIV-1-which is sufficient to drive the assembly of virus-like particles-with the use of fluorescence resonance energy transfer, fluorescence recovery after photobleaching and total-internal-reflection fluorescent microscopy in living cells. Virions appeared individually at the plasma membrane, their assembly rate accelerated as Gag protein accumulated in cells, and typically 5-6 min was required to complete the assembly of a single virion. These approaches allow a previously unobserved view of the genesis of individual virions and the determination of parameters of viral assembly that are inaccessible with conventional techniques.

Imaging of Rab5 activity identifies essential regulators for phagosome maturation

Masahiro Kitano — 2008-04-03T16:42:31-00:00

Nature (02 April 2008)

Efficient phagocytosis of apoptotic cells is crucial for tissue homeostasis and the immune response. Rab5 is known as a key regulator of the early endocytic pathway and we have recently shown that Rab5 is also implicated in apoptotic cell engulfment; however, the precise spatio-temporal dynamics of Rab5 activity remain unknown. Here, using a newly developed fluorescence resonance energy transfer biosensor, we describe a change in Rab5 activity during the engulfment of apoptotic thymocytes. Rab5 activity on phagosome membranes began to increase on disassembly of the actin coat encapsulating phagosomes. Rab5 activation was either continuous or repetitive for up to 10 min, but it ended before the collapse of engulfed apoptotic cells. Expression of a dominant-negative mutant of Rab5 delayed this collapse of apoptotic thymocytes, showing a role for Rab5 in phagosome maturation. Disruption of microtubules with nocodazole inhibited Rab5 activation on the phagosome membrane without perturbing the engulfment of apoptotic cells. Furthermore, we found that Gapex-5 is the guanine nucleotide exchange factor essential for Rab5 activation during the engulfment of apoptotic cells. Gapex-5 was bound to a microtubule-tip-associating protein, EB1, whose depletion inhibited Rab5 activation during phagocytosis. We therefore propose a mechanistic model in which the recruitment of Gapex-5 to phagosomes through the microtubule network induces the transient Rab5 activation.

Flotillin-dependent clustering of the amyloid precursor protein regulates its endocytosis and amyloidogenic processing in neurons.

A Schneider — 2008-04-23T08:17:58-00:00

The Journal of neuroscience : the official journal of the Society for Neuroscience, Vol. 28, No. 11. (12 March 2008), pp. 2874-2882.

The flotillins/reggie proteins are associated with noncaveolar membrane microdomains and have been implicated in the regulation of a clathrin- and caveolin-independent endocytosis pathway. Endocytosis is required for the amyloidogenic processing of the amyloid precursor protein (APP) and thus to initiate the release of the neurotoxic beta-amyloid peptide (Abeta), the major component of extracellular plaques found in the brains of Alzheimer's disease patients. Here, we report that small interference RNA-mediated downregulation of flotillin-2 impairs the endocytosis of APP, in both neuroblastoma cells and primary cultures of hippocampal neurons, and reduces the production of Abeta. Similar to tetanus neurotoxin endocytosis, but unlike the internalization of transferrin, clathrin-dependent endocytosis of APP requires cholesterol and adaptor protein-2 but is independent of epsin1 function. Moreover, on a nanoscale resolution using stimulated emission depletion microscopy and by Förster resonance energy transfer with fluorescence lifetime imaging microscopy, we provide evidence that flotillin-2 promotes the clustering of APP at the cell surface. We show that the interaction of flotillin-2 with APP is dependent on cholesterol and that clustering of APP enhances its endocytosis rate. Together, our data suggest that cholesterol/flotillin-dependent clustering of APP may stimulate the internalization into a specialized clathrin-dependent endocytosis pathway to promote amyloidogenic processing.

Visualization of AP-1 NF-kappaB ternary complexes in living cells by using a BiFC-based FRET

John Shyu — 2008-01-03T08:10:19-00:00

Proceedings of the National Academy of Sciences (2 January 2008), 0705181105.

Proteinprotein interactions are essential for maintaining cell structure and for executing almost all cellular processes. Determination of where and how each protein interacts with its partners provides significant insight into proteins' cellular roles. Although several assays, such as FRET and bimolecular fluorescence complementation (BiFC), have been developed and widely used for visualization and identification of protein interactions in living cells, there is no simple and convenient assay to visualize and identify multiple protein complexes in living cells. Because many signaling molecules often function as ternary complexes, availability of an assay for visualization and identification of ternary complexes will significantly expand the repertoire of protein interaction studies in living cells. By using the FosJunnuclear factor of activated T cells (NFAT) ternary complex as a model and the fluorescent proteins Cerulean and Venus, two mutant proteins of CFP and YFP with better folding and less environment sensitivity, as a donor and acceptor, respectively, we have combined a Venus-based BiFC system with Cerulean to develop a BiFC-based FRET (BiFC-FRET) assay for visualization of ternary complexes in living cells with a conventional three-filter FRET setup. We also have applied the BiFC-FRET to identify a ternary complex formed between FosJun heterodimers and the NF-kappaB subunit, p65. This finding reveals a cross-talk between AP-1 and NF-kappaB. Thus, the BiFC-FRET represents a convenient assay for identification and visualization of ternary complexes in living cells. 10.1073/pnas.0705181105

Fluorescent protein FRET: the good, the bad and the ugly

David Piston — 2007-09-17T10:26:58-00:00

Trends in Biochemical Sciences, Vol. 32, No. 9. (September 2007), pp. 407-414.

Dynamic protein interactions play a significant part in many cellular processes. A technique that shows considerable promise in elucidating such interactions is Forster resonance energy transfer (FRET). When combined with multiple, colored fluorescent proteins, FRET permits high spatial resolution assays of protein-protein interactions in living cells. Because FRET signals are usually small, however, their measurement requires careful interpretation and several control experiments. Nevertheless, the use of FRET in cell biological experiments has exploded over the past few years. Here we describe the physical basis of FRET and the fluorescent proteins appropriate for these experiments. We also review the approaches that can be used to measure FRET, with particular emphasis on the potential artifacts associated with each approach.