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Enveloped viruses deliver their virions into the host cell by fusion with the cellular plasma or endosomal membrane, thus creating topological continuity between the cytosol and the inside of the viral envelope. Nonenveloped viruses are, by their very nature, denied this strategy and must employ alternative methods to breach their host cell's delimiting membrane. We show here that the compact icosahedral parvoviral virion gains entry by deploying a lipolytic enzyme, phospholipase A2 (PLA2), that is expressed at the N terminus of VP1, the minor coat protein. This region of VP1 is normally sequestered within the viral shell but is extruded during the entry process as a capsid-tethered domain. A single amino acid substitution in the active site of the VP1 PLA2 inactivates enzymatic activity and abrogates infectivity. We have used transencapsidation of a vector expressing green fluorescent protein to show that infection by this PLA2-defective mutant can be complemented by coinfection with wild-type or mutant full virions, provided they can express a functional PLA2. Even though wild- type empty capsids contain an active form of the enzyme, it is not externalized under physiological conditions, and such capsids are not able to complement the PLA2 mutant. Significantly, highly efficient rescue can be achieved by polyethyleneimine-induced endosome rupture or by coinfection with adenovirus as long as uptake of the two viruses is simultaneous and the adenovirus is capable of deploying pVI, a capsid protein with endosomolytic activity. Together, these results demonstrate a previously unrecognized enzymatic mechanism for nonenveloped virus penetration. [ABSTRACT FROM AUTHOR]