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They are notorious for being difficult to isolate and study, and mimicking native conditions for studies in vitro has proved to be a challenge. Lipid nanodiscs are among the most promising platforms for MP reconstitution, but they contain a relatively labile lipid bilayer and their use requires previous protein solubilization in detergent. These limitations have led to the testing of copolymers in new types of nanodisc platforms. Polymer-encased nanodiscs and polymer nanodiscs support functional MPs and address some of the limitations present in other MP reconstitution platforms. In this review, we provide a summary of recent developments in the use of polymers in nanodiscs. MPs are essential for cell homeostasis, signaling, transport, and energy production. Membranes and Their Functions
Membranes and Their Functions Video
Cell membrane introduction - Cells - MCAT - Khan AcademyJoe Grove, Mark Marsh; The cell biology of receptor-mediated virus entry. J Cell Biol 26 December ; 7 : — The cell imposes multiple barriers to virus entry. However, viruses exploit fundamental cellular processes to gain entry to cells and deliver their genetic cargo.
Virus entry pathways are largely defined by the interactions link virus particles and their receptors at the cell surface. These interactions determine the mechanisms of Membfanes attachment, uptake, intracellular trafficking, and, ultimately, penetration to the cytosol. Elucidating the complex interplay between viruses and their receptors is necessary for a full understanding of how these remarkable agents invade their cellular hosts.
Within an infected cell, viral nucleic acid, Membraes it RNA or DNA, is relatively cosseted by cellular membranes and a protective cytosolic environment, but the cell-free stage that viral genomes must transit to access new host cells is fraught with danger. Thus, the unlocking process is Membranes and Their Functions directly coupled to the mechanisms through which viral genomes are transferred across a limiting cellular membrane usually the plasma membrane or endosome membranethe principal barrier to infection.
Introduction
In this review, we Fnctions how events at the cell surface determine viral entry pathways and, using several different examples, examine some of the strategies viruses use to overcome cellular barriers to infection Fig. Receptor-mediated signaling will emerge throughout the review as Membranes and Their Functions important component of virus entry that can operate at multiple stages, as will insights into the variations that viruses have developed on the principle themes for entry.
These first interactions, which may lack specificity, are often electrostatic and serve primarily to give a virus an initial catch-hold from which it can then recruit specific receptors that drive the reactions leading to entry. The receptors are cell surface molecules that provide functions essential for productive infection.
In simple situations, receptors can efficiently target viruses for endocytosis Fig. In other cases, the reasons underlying the use of specific receptors are more obscure, and a full appreciation will probably require better understanding of the mode of entry of the virus into the hosts, Mmebranes architecture of target cells within different tissue environments, and the biology of the virus within its hosts. The use of specific cell surface components with restricted expression patterns is Theit responsible for viral tropism, i. A number of the cell surface components exploited by viruses have now been identified Table I.
Many viruses use single molecular species as receptors, for example CD Membranes and Their Functions poliovirus Mendelsohn et al. However, other viruses exhibit a more complex receptor dependency that involves engagement with at least two distinct plasma membrane components, each of which Tax Income essential Fig.
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Human immunodeficiency viruses HIVs are the archetype for such a process. By Membranes and Their Functions constraints that prevent Env from transitioning to thermodynamically more stable conformations, these interactions initiate conformational changes that facilitate strain-specific interactions of read more with the coreceptors CCR5 or CXCR4 and allow initial structural changes in gp41, the Env component that promotes fusion Choe et al. Engagement of the coreceptor drives further Env structural rearrangements that culminate in fusion of the viral and host membranes Fig.
Another intriguing example of a virus requiring multiple cell surface components for entry is hepatitis C virus HCV. In addition to these key components, the minimal HCV entry complex requires the tight junction components claudin-1 and occludin. There is limited evidence for a direct interaction between the HCV glycoproteins and claudin-1 or occludin Evans et al. HCV requires clathrin-mediated endocytosis and low endosomal pH for productive infection Blanchard et al.
The Membrane Of The Cell Membrane
That HCV uses such a complex array of cell surface components to achieve this goal suggests that receptor engagement plays a more substantial role in virus entry than just guiding virions into clathrin-coated vesicles. Although viruses have the potential to cluster homogenous or heterogenous receptors, we know relatively little of the stoichiometry of receptor engagement.
Lateral motion on the cell surface or along filopodia has been observed for several viruses and may help viruses encounter necessary coreceptors in numbers sufficient to generate productive entry events Lehmann et al.]
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