Mimivirus giant particles incorporate a large fraction of anonymous and unique gene products.

Acanthamoeba polyphaga mimivirus is the largest known virus in both particle size and genome complexity. Its 1.2-Mb genome encodes 911 proteins, among which only 298 have predicted functions. The composition of purified isolated virions was analyzed by using a combined electrophoresis/mass spectrometry approach allowing the identification of 114 proteins. Besides the expected major structural components, the viral particle packages 12 proteins unambiguously associated with transcriptional machinery, 3 proteins associated with DNA repair, and 2 topoisomerases. Other main functional categories represented in the virion include oxidative pathways and protein modification. More than half of the identified virion-associated proteins correspond to anonymous genes of unknown function, including 45 "ORFans." As demonstrated by both Western blotting and immunogold staining, some of these "ORFans," which lack any convincing similarity in the sequence databases, are endowed with antigenic properties. Thus, anonymous and unique genes constituting the majority of the mimivirus gene complement encode bona fide proteins that are likely to participate in well-integrated processes.