Shedding light on the multiple functions of the geminivirus Replication initiator protein

Geminiviruses are single-stranded DNA viruses that infect many plant species and cause extensive crop losses worldwide. To attain durable and broad resistance against this devastating virus family, the most conserved geminiviral protein, Replication initiator protein (Rep), was here studied. Rep is a multifunctional protein that is essential for viral replication. It orchestrates all layers of the viral DNA replication process by interacting with numerous host proteins. This thesis describes how Rep interferes with SUMO conjugation, an essential protein modification that regulates the nuclear activity of hundreds of plant proteins. Data is presented that Rep modulates the biological activity of one particular host target, Proliferating cell nuclear antigen (PCNA), by suppressing PCNA sumoylation while promoting its ubiquitination. Moreover, two different regions in Rep were identified that are necessary for its nuclear localization and its interaction with SUMO, respectively. In agreement, Rep resides in a complex together with SUMO and the SUMO E2 conjugating enzyme (SCE1) and this complex aggregates in novel subnuclear compartments (NBs). These interactions with SUMO/SCE1 were also essential for viral replication activity of Rep. These NBs co-localize with a second nuclear compartment: Photobodies. Photobodies contain proteins involved in light perception and plant growth in response to environmental cues. Remarkably, blue light triggers a relocalization of Rep towards NBs that overlap with nuclear compartments that contain blue light receptors. Finally, using affinity purification, novel host proteins were identified that interact with Rep. These findings will help to develop novel strategies to obtain resistance towards Geminiviruses.