The complex infection process of parvoviruses is not well understood so far. An important role has been attributed to a phospholipase A 2 domain which is located within the unique N terminus of the capsid protein VP1. Based on the structural difference between adeno-associated virus type 2 wild-type capsids and capsids lacking VP1 or VP2, we show via electron cryomicroscopy that the N termini of VP1 and VP2 are involved in forming globules inside the capsids of empty and full particles. Upon limited heat shock, VP1 and possibly VP2 become exposed on the outsides of full but not empty capsids, which is correlated with the disappearance of the globules in the inner surfaces of the capsids. Using molecular modeling, we discuss the constraints on the release of the globularly organized VP1-unique N termini through the channels at the fivefold symmetry axes outside of the capsid.For infection, nonenveloped viruses are involved in a number of interactions with macromolecular structures of the host cell that enable virus uptake, disassembly, and delivery of the genome. Such interactions not only implicate recognition of key elements required for cell entry and intracellular passage but also enable activities of viral proteins to overcome the barriers of the host cell. This is exemplified by the binding of viral capsids to cellular receptors, the active release from endocytic vesicles, the use of cellular filament systems for trafficking, and different kinds of delivery of the viral genomes to the cell nucleus (30, 60). These processes are often supported by signaling events which reflect the reaction of the cell to the attachment and uptake of the virus (16). The information for these interactions is maintained within the structure of the virus shell.The infection process of adeno-associated virus type 2 (AAV-2) is initiated by attachment to the cell surface via binding to heparan-sulfate proteoglycan (53). Upon interaction with a secondary receptor, either fibroblast growth factor receptor 1 or ␣V5 integrin (41, 52), virions enter the cell by means of clathrin-coated pits (5) into the endosomal pathway. While a large number of viral particles seem to accumulate in perinuclear vesicular compartments (4), they have to escape into the cytoplasm for successful infection (21, 63). Rac signaling and interaction with cellular filament systems have an impact on trafficking toward the nucleus (44). There is increasing evidence that virions can enter the cell nucleus (5, 45, 63), possibly by an as yet unknown pathway which is independent of passage through the nuclear pores (22, 63). According to this scenario, the final uncoating reaction has to take place in the nucleus.The AAV-2 capsid harbors a 4.7-kb linear single-stranded genome which contains two open reading frames coding for four nonstructural proteins and three capsid proteins (48). They are flanked by two identical inverted terminal repeats. The nonstructural proteins have functions in the control of gene expression, in DNA replication, and in genome encapsidation (...
Adeno-associated virus type 2 empty capsids are composed of three proteins, VP1, VP2 and VP3, which have relative molecular masses of 87, 72 and 62 kDa, respectively, and differ in their N-terminal amino acid sequences. They have a likely molar ratio of 1:1:8 and occupy symmetrical equivalent positions in an icosahedrally arranged protein shell. We have investigated empty capsids of adeno-associated virus type 2 by electron cryo-microscopy and icosahedral image reconstruction. The three-dimensional map at 1.05 nm resolution showed sets of three elongated spikes surrounding the three-fold symmetry axes and narrow empty channels at the five-fold axes. The inside of the capsid superimposed with the previously determined structure of the canine parvovirus (Q. Xie and M.S. Chapman, 1996, J. Mol. Biol., 264, 497-520), whereas the outer surface showed clear discrepancies. Globular structures at the inner surface of the capsid at the two-fold symmetry axes were identified as possible positions for the N-terminal extensions of VP1 and VP2.
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