Geminiviruses constitute a large group of plant virus whose genome is packed as single-stranded DNA circles in a twinned isometric particle and is converted to double-stranded forms in nuclei of differentiated plant cells. Members of the genus Begomovirus, such as Cabbage leaf curl virus (CaLCuV), possess two genomic components, DNA-A and DNA-B. The DNA-A is involved in replication, transcriptional activation of viral genes and encapsidation of the viral genome. The DNA-B encodes two movement proteins, the Movement Protein (MP) and the Nuclear Shuttle Protein (NSP), both required for systemic infection. NSP shuttles the viral DNA between the nucleus and the cytoplasm and then acts cooperatively with MP to move the viral DNA cell-to-cell across the wall. The localization of NSP and its proposed role in cell-to-cell movement of the viral DNA predict that interactions with host factors may occur in both the cytoplasm and the nucleus. A proline-rich extensin-like receptor protein kinase (PERK) was found to interact specifically with NSP of CaLCuV and of tomato-infecting geminiviruses, through yeast two-hybrid screening. The PERK-like protein, which we designated NsAK (NSP-associated kinase), is structurally organized into a proline-rich N-terminal domain followed by a transmembrane segment and a C-terminal serine/threonine kinase domain. The viral protein interacted stably with defective versions of the NsAK kinase domain but not with the potentially active enzyme in an in vitro binding assay. In vitro translated NsAK enhanced the phosphorylation level of NSP, indicating that NSP functions as substrate for NsAK. These results demonstrated that NsAK is an authentic serine/threonine kinase and suggested a functional link for the NSP-NsAK complex formation. This interpretation was corroborated by in vivo infectivity assays showing that loss of NsAK function delays the onset of CaLCuV infection and attenuates symptom development. Our data implicate NsAK as a positive contributor to geminivirus infection and suggest it may regulate NSP function.
Supported by CAPES, CNPq and FAPEMIG.