Influenza A virus nucleoprotein targets subnuclear structures.

The Influenza A virus nucleoprotein (NP) is the major protein component of the genomic viral ribonucleoprotein (vRNP) complexes, which are the replication- and transcription-competent units of Influenza viruses. Early during infection, NP mediates import of vRNPs into the host cell nucleus where viral replication and transcription take place; also newly synthesized NP molecules are targeted into the nucleus, enabling coreplicational assembly of progeny vRNPs. NP reportedly acts as regulatory factor during infection, and it is known to be involved in numerous interactions with host cell proteins. Yet, the NP-host cell interplay is still poorly understood. Here, we report that NP significantly interacts with the nuclear compartment and displays distinct affinities for different subnuclear structures. NP subnuclear behavior was studied by expression of fluorescent NP fusion proteins - including obligate monomeric NP - and site-specific fluorescence photoactivation measurements. We found that NP constructs accumulate in subnuclear domains frequently found adjacent to or overlapping with promyelocytic leukemia bodies and Cajal bodies. Targeting of NP to Cajal bodies could further be demonstrated in the context of virus infection. We hypothesize that by targeting functional nuclear organization, NP might either link viral replication to specific cellular machinery or interfere with host cell processes.

Influenza A matrix protein M1 multimerizes upon binding to lipid membranes.

The matrix protein M1 plays a pivotal role in the budding of influenza virus from the plasma membrane (PM) of infected cells. This protein interacts with viral genetic material and envelope proteins while binding to the inner leaflet of the PM. Its oligomerization is therefore closely connected to the assembly of viral components and the formation of new virions. Of interest, the molecular details of M1 interaction with lipids and other viral proteins are far from being understood, and it remains to be determined whether the multimerization of M1 is affected by its binding to the PM and interaction with its components. To clarify the connection between M1 oligomerization and binding to lipid membranes, we applied a combination of several quantitative microscopy approaches. First, we used number and brightness (N&B) microscopy to characterize protein multimerization upon interaction with the PM of living cells. Second, we used controlled biophysical models of the PM (i.e., supported bilayers) to delve into the details of M1-lipid and M1-M1 interactions by employing a combination of raster image correlation spectroscopy (RICS), fluorescence correlation spectroscopy (FCS), and atomic force microscopy (AFM). Our results show that M1 oligomer formation is strongly enhanced by membrane binding and does not necessarily require the presence of other viral proteins. Furthermore, we propose a specific model to explain M1 binding to the lipid bilayer and the formation of multimers.

pH-Controlled two-step uncoating of influenza virus.

Upon endocytosis in its cellular host, influenza A virus transits via early to late endosomes. To efficiently release its genome, the composite viral shell must undergo significant structural rearrangement, but the exact sequence of events leading to viral uncoating remains largely speculative. In addition, no change in viral structure has ever been identified at the level of early endosomes, raising a question about their role. We performed AFM indentation on single viruses in conjunction with cellular assays under conditions that mimicked gradual acidification from early to late endosomes. We found that the release of the influenza genome requires sequential exposure to the pH of both early and late endosomes, with each step corresponding to changes in the virus mechanical response. Step 1 (pH 7.5-6) involves a modification of both hemagglutinin and the viral lumen and is reversible, whereas Step 2 (pH <6.0) involves M1 dissociation and major hemagglutinin conformational changes and is irreversible. Bypassing the early-endosomal pH step or blocking the envelope proton channel M2 precludes proper genome release and efficient infection, illustrating the importance of viral lumen acidification during the early endosomal residence for influenza virus infection.

Uptake of a fluorescent methyl-ß-cyclodextrin via clathrin-dependent endocytosis.

Cyclodextrins (CDs) are widely used both in pharmaceutical applications to improve drug bioavailability and in cell biology as cholesterol-depleting and -delivering agents. Recently, it was shown that ß-CD covalently coupled to fluorescent dextran polymers accumulates in cholesterol-enriched lysosomal storage organelles of human fibroblasts (Rosenbaum et al., 2010). By employing a methyl-ßCD tagged with fluorescein (FMßCD), we have characterized the cellular trafficking of the CD in mammalian cell lines and its distribution into the endocytic compartments within the first minutes following addition to cells. FMßCD enters mammalian cells via endocytosis. The colocalization of FMßCD with transferrin-containing endosomes and the inhibition of FMßCD internalization by chlorpromazine or by an antisense RNA against clathrin heavy chain indicate that FMßCD is taken up via receptor-mediated, clathrin-dependent endocytosis. These results not only highlight the possibility of using CDs to target drugs intracellularly, but also warn about potential unwanted effects on cell physiology other than cholesterol extraction/loading at high concentrations, high temperatures and prolonged incubation times.

New molecular rods--characterization of their interaction with membranes.

Molecular rods are synthetical molecules consisting of a hydrophobic backbone which are functionalized with varying terminal groups. Here, we report on the interaction of a recently described new class of molecular rods with lipid and biological membranes. In order to characterize this interaction, different fluorescently labeled rods were synthesized allowing for the application of fluorescence spectroscopy and microscopy based approaches. Our data show that the rods are incorporated into membranes with a perpendicular orientation to the membrane surface and enrich preferentially in liquid-disordered lipid domains. These characteristics underline that rods can be applied as stable membrane-associated anchors for functionalizing membrane surfaces.

Use of liposomes for studying interactions of soluble proteins with cellular membranes.

Methods are described that have been used for characterizing the interaction of the soluble bovine seminal plasma protein PDC-109 with liposomes. PDC-109 binds to bull sperm cells upon ejaculation and is an important modulating factor of sperm cell maturation. The binding of the protein to sperm cells is mediated via lipids of the sperm plasma membrane. Most of our current knowledge about the molecular mechanisms of PDC-109-membrane interaction has been obtained by studies employing lipid vesicles. The experimental strategy described here can be applied to investigate the interaction of soluble proteins with membranes in order to understand their physiological role.