A Peculiar Phospholipid-Binding Specificity and Cell Death-Related Properties of Tubic (Nicotiana tabacum Tubby-like F-Box Protein 8)

Proteins possessing the Tubby domain (Tubby-like proteins, TLPs) are widespread in plants and represented by protein families. Although the function of TLPs is frequently associated with plant response to stresses, the mechanisms of their action are largely unknown. The recently demonstrated interaction of Tubic (Nicotiana tabacum Tubby-like F-box protein 8) with a cell death protease, phytaspase, prompted us to investigate the possible involvement of Tubic in plant cell death and to perform structural and functional characterization of distinct domains of Tubic. We demonstrated here a requirement for Tubic in the execution of the hypersensitive response of Nicotiana benthamiana plants induced by the INF1 protein of phytophthora. Also in line with the pro-death properties of Tubic was the observation that prolonged overexpression of Tubic compromised plant viability. Notably, inactivating mutations or complete deletion of the F-box domain markedly enhanced the cell death-inducing activity of Tubic. The N domain of Tubic turned out to be necessary for the observed down-regulation of phytaspase activity by Tubic overproduction. Cooperation between the N domain and the Tubby domain was apparently required to achieve maximum (approximately 2-fold) suppressive effect. In turn, for the Tubby domain, an expected capability to bind phospholipids was demonstrated, yet the specificity of this interaction was quite distinct from that reported for plant Tubby domains previously, with phosphatidic acid and phosphatidylinositol monophosphates being by far the strongest interactors. We propose that dissimilarity in phospholipid-binding specificity between the TLP family members may underlie the diversity in their responses to external cues.