Cell entry

Protein toxins acting intracellularly are often composed of two subunits (A/B model). One subunit is catalytic (A-subunit) and the other is responsible for binding and cell entry (B-subunit). Following binding to an extracellular membrane receptor, the toxins are endocytosed. From the endosomes, the A-subunit is directly (pH dqDendent) transferred into the cytosol (e.g., diphtheria toxin and anthrax toxin) or the toxin is transported in a retrograde manner via the golgi to the ER (e.g., cholera toxin), where translocation into the cytosol occurs [1]. 

Platt EJ, Dumin IP, Rabat D (2005) Kinetic factors control efficiencies of cell entry, efficacies of entry inhibitors, and mechanisms of adaptation of human immunodeficiency virus, J Virol 79 4347 356... 

Hofer F. Members of the low density lipoprotein receptor family mediate cell entry of a minor group common cold virus. Proc Natl Acad Sci USA 1994 91 1839-1842. 

Colombatti, M., Greenfield, L., and Youle, R.J. (1986) Cloned fragment of diphtheria toxin linked to T cell-specific antibody identifies regions of B chain active in cell entry./. Biol. Ghem. 261, 3030. 

To determine if the high in vitro potents of the anti-HIV compound 30 translates into antiviral efficiency in vivo, Datema et al. investigated the inhibition of HIV-1 production and of depletion of human T cells in HIV-1-infected SCID-hu Thy/Liv mice [37]. Steady levels of 100 ng of 30 or higher per mL in plasma resulted in significant inhibition of HIV p24 protein formation. Daily injection of 30 caused a dose-dependent decrease in viral p24 production, and this inhibition could be potentiated by coadministration of AZT (or DDI). This study suggested that 30 alone or in combination with the licensed anti-HIV agents AZT and DDI may decrease the virus load in HIV-infected patients and, by extension, that the infectious cell entry step is a valid target for antiviral chemotherapy of HIV disease. 

At this point, the acyl-CoA is still in the cytosol of the muscle cell. Entry of the acyl-CoA into the mitochondrial matrix requires two translocase enzymes, carnitine acyl transferase I and carnitine acyl transferase II (CAT I and CAT II), and a carrier molecule called carnitine the carnitine shuttles between the two membranes. The process of transporting fatty acyl-CoA into mitochondria is shown in Figure 7.15. 

Integrin receptor-binding peptides have been used to enhance liposome binding, uptake, and expression (25,47 9). The inclusion of an 0(5pi integrin-targeted peptide into a liposomal complex enhanced transfection efficiency four- to five-fold in Jurkat cells and 10- to 13-fold in TF-1 cells (48). Confocal and electron microscopy revealed that the mechanism of cell entry conferred by RGD peptides on liposomes is predominantly by clathrin-coated endocytosis rather than by phagocytosis (50). 

Figure 2 Proposed pathways for liposomal entry into the cell enhanced by peptides. These include direct cell entry suggested as the mechanism of entry by cell-penetrating peptides and receptor-mediated endocytosis by caveolae- and clathrin-dependent endocytosis.

TAT liposomes remain intact within one hour of translocation and slowly migrate through the cell, bypassing the endocytic pathway, to the perinuclear zone where they disintegrate (95). The mechanism utilized by TAT to migrate across the membrane was thought to be energy independent because it operates at similar rates at both 4°C and 37°C (95,96). Cell entry by TAT is also unhindered by metabolic inhibitors such as sodium azide or iodoacetamide (97). Peptides constructed of both the d and l amino acids of Antp can be detected intracellularly, the inference of which is that no specific receptor was required because both isomers had equal potential (98,99). 

MuLVs assayed, it must use a unique receptor for cell entry. However, as ecotropic ... 

Miller, A. and Chen, F. (1996) Retrovirus packaging cells based on lOAl murine leukemia virus for production of vectors that use multiple receptors for cell entry. J VlrollO, 5564-5571. 

Miller, D. and Miller, A. (1994) A family of retroviruses that utilize related phosphate transporters for cell entry. J Virol53, 8270-8276. 

Ross, S., Schofield, J., Farr, C. and Bucan, M. (2002) Mouse transferrin receptor 1 is the cell entry receptor for mouse mammary tumor virus. Proc. Natl. Acad. Sci, USA%%, 12386-12390. 

Different AAV serotypes have shown remarkably different expression patterns because of differences in cell entry and intracellular activities (66, 67). Application of the dimerizer-inducible transcriptional regulatory system for AAV has allowed pharmacological regulation of heterologous gene expression in vivo (68). 

Phospholipids are ideal compounds for making membranes because of their amphipathic nature (see chapter 17). The polar head-groups of phospholipids prefer an aqueous environment, whereas the nonpolar acyl substituents do not. As a result, phospholipids spontaneously form bilayer structures (see fig. 17.6), which are a dominant feature of most membranes. The phospholipid bilayer is the barrier of the cell membrane that prevents the unrestricted transport of most molecules other than water into the cell. Entry of other molecules is allowed if a specific transport protein is present in the cell membrane. Similarly, the phospholipid bilayer prevents leakage of metabolites from the cell. The amphipathic nature of phospholipids has a great influence on the mode of their biosynthesis. Thus, most of the reactions involved in lipid synthesis occur on the surface of membrane structures catalyzed by enzymes that are themselves amphipathic. 

Kuwajerwala N, Cifuentes E, Gautam S, Menon M, Barrack ER, Reddy GP. 2002. Resveratrol induces prostate cancer cell entry into S phase and inhibits DNA synthesis. Cancer Res 62 2488-2492. 

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