Proceso de Adsorción y Liberación de Virus en Células: Enfoque pH-Dependiente - Prof. Giro, Apuntes de Virología

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Rosina Girones

TEMAS 3 Y 4. CICLO DE REPLICACIÓN

Í N D I C E

Generalidades de la multiplicación de los virus. Restricciones causadas por las células huésped. Multiplicación de virus DNA y de virus RNA. Estrategias replicativas. Interacciones virus-célula.

REPLICACIÓN DE VIRUS: CURVA “ ONE-STEP GROWTH ”

REPLICACIÓN GENERAL DE VIRUS

Diversity of viral replication: example of Ebolavirus versus Herpesvirus.

Hulo C et al. Nucl. Acids Res. 2011;39:D576-D

© The Author(s) 2010. Published by Oxford University Press. (^) TEMA 3 Y 4

RECEPTORES CELULARES

ADSORCIÓN DE LOS VIRUS A LAS CÉLULAS The uptake of unenveloped viruses was, until recently, always considered to involve endocytosis into an endosome, but other mechanisms have been also identified. An example is the poliovirus which belongs to the picornavirus family. This is an icosahedral T=3 virus. The capsid consists of 4 polypeptides, VP1, 2, 3 and 4. VP4 is buried in the capsid and associated with the viral RNA. VP1, 2 and 3 form the external capsid. Five VP1 protein subunits at the 5 fold axis of symmetry form a canyon in the capsid which is the recognition site for the receptor. The polio receptor belongs to IgG superfamily (as does CD4), it has a MW of 45Kd. It has an amino terminal Ig-like domain which binds to the virus, a transmembrane domain and a cytoplasmic domain (which is dispensable for function). There are several thousand copies of the receptor present on a cell.

ADSORCIÓN DE LOS VIRUS A LAS CÉLULAS After binding to the receptor, the virus is endocytosed into an endosome. At a low pH, poliovirus becomes more lipophilic and can associate with the endosome membrane possibly forming a pore. Presumably the pH shift results in exposure of hydrophobic domains in the capsid proteins. During this process VP4 is lost from the particle; VP2 may also be lost. This gives the particle increased flexibility. The genomic RNA is ejected through an endosomal pore into the cytoplasm. It is possible that poliovirus can also enter the cytoplasm directly in a pH- independent process. It has been demonstrated that the purified viral receptor converts the 160S poliovirus particle into a 135S form lacking VP4. This further dissociates to an 80S complex in which the RNA has been lost. It has also been observed that an "engineered" receptor molecule which lacks the cytoplasmic domain and in its place contains a GPI anchor preventing internalization still confers susceptibility to infection. It is possible that the hydrophobic amino terminus of VP1 is exposed on the surface of the capsid during the transition from a 160S complex to a 135S complex and that the capsid then fuses directly with the cell membrane.

ADSORCIÓN DE LOS VIRUS A LAS CÉLULAS

The receptor for HIV is CD4 antigen, which binds gp120 with an affinity constant in the

nanomolar range. Binding has been demonstrated by coprecipitation experiments using

antibodies to either protein.

Definitive proof that CD4 is the HIV receptor was obtained by Maddon and Weiss. They

showed that CD4 negative human cells (e.g. HeLa cells) cannot be infected with HIV.

Expression of CD4 in these cells following transfection with a cloned CD4 gene made

them permissive. However, additional structures are also necessary, since transfection of

mouse cells with CD4 does not render them permissive for HIV infection. These second

receptor components have recently been identified as members of the beta-chemokine

family, in particular CXCR-4 and CCR-5.

www.microbiologybytes.com/virology

ENTRADA POR TRANSLOCACIÓN

ENTRADA POR: A) PENETRACIÓN DIRECTA, B) FUSIÓN DE MEMBRANA

ENTRADA POR ENDOCITOSIS Y FUSIÓN DEPENDIENTE DE PH