Crystal structure of phyllogen， a phyllody-inducing effector protein of phytoplasma.

Phytoplasmas are plant pathogenic bacteria that often induce unique phyllody symptoms in which the floral organs are transformed into leaf-like structures. Recently， a novel family of bacterial effector genes， called phyllody-inducing genes (phyllogens)， was identified as being involved in the induction of phyllody by degrading floral MADS-domain transcription factors (MTFs). However， the structural characteristics of phyllogens are unknown. In this study， we elucidated the crystal structure of PHYL1， a phyllogen of 'Candidatus Phytoplasma asteris' onion yellows strain， at a resolution of 2.4 Å. The structure of PHYL1 consisted of two α-helices connected by a random loop in a coiled-coil manner. In both α-helices， the distributions of hydrophobic residues were conserved among phyllogens. Amino acid insertion mutations into either α-helix resulted in the loss of phyllody-inducing activity and the ability of the phyllogen to degrade floral MTF. In contrast， the same insertion in the loop region did not affect either activity， indicating that both conserved α-helices are important for the function of phyllogens. This is the first report on the crystal structure of an effector protein of phytoplasmas.