Nuclear localization of the Tomato bushy stunt virus cell-to-cell movement protein P22 and its interactive host protein HFi22

The Tomato bushy stunt virus (TBSV) p22 protein (P22) has intrinsic biochemical properties that are typical for viral cell-to-cell movement proteins. In addition, P22 specifically interacts with a homeodomain host transcription factor (HFi22). Furthermore, P22 is also known to serve as an elicitor of a hypersensitive resistance response in Nicotiana edwardsonii. Sub-cellular fractionation studies show that HFi22 accumulates predominantly in the nucleus, although upon infection with TBSV it is also detectable in the cytoplasm. Recently we observed that P22 not only localizes to plasmodesmata but unexpectedly it also accumulates in the nucleus. This property is controlled by a carboxyl terminal nuclear localization signal (NLS) on P22. Mutations in the NLS that compromise the nuclear localization of P22 prevent the induction of necrotic local lesions typically induced by wild-type P22 on N. edwardsonii. These findings lend support to the hypothesis that the P22-HFi22 interaction in the nucleus may cause the activation of defense related genes.