Structural analysis of phyllogen, a phyllody-inducing effector, revealed the importance of two conserved-helices

Phyllody is one of the most characteristic symptoms associated with phytoplasmas, in which flower organs are transformed into leaf-like structures. A phytoplasma conserved effector, called phyllogen (phyllody-inducing gene family), works as a phyllody inducer by degrading floral MADS-domain transcription factors (MTFs). In this study the crystal structure of PHYL1_OY, a phyllogen of ‘Candidatus Phytoplasma asteris’ onion yellows strain, was elucidated at a resolution of 2.4 Å. PHYL1_OY, is formed by two α-helices connected by a random loop in a coiled-coil manner. Other phyllogens were also predicted to contain two consensus α-helices. Amino acid insertion mutations into either α-helix of PHYL1_OY disrupted its phyllody-inducing activity and ability to degrade MTF, although the same insertion in the loop region did not affect either. These results indicated that both conserved α-helices are important for the function of phyllogen.