Families of ubiquitin E3-ligase mimicking effector proteins in the genome of Hessian fly (Mayetiola destructor)

Hessian fly (HF) feeding alters the development of its host plant wheat (Triticum spp.).  Nutrient tissue forms at the feeding site and wheat plants become permanently stunted. Recent evidence indicates that effector proteins secreted in salivary fluids are responsible for these changes in plant development.  More than 800 putative effector proteins have been identified in the HF genome. Together, secreted salivary gland protein family 71 (SSGP-71) and SSGP family 16 (SSGP-16), account for over 500 of these genes.  Protein sequence and predicted structural analyses indicated that family SSGP-71 genes encode mature proteins with an N-terminal F-box domain followed by a series of leucine-rich repeats (LRR). These structures are analogous to motifs present in plant F-box E3 ligases. Similar analyses suggest that at least 10 members of family SSGP-16 encode proteins that contain a RING-finger motif.  These structures are analogous to motifs present in RING-type plant E3 ligases. Yeast-two hybrid protein-protein interaction assays demonstrated that the F-box motif of one SSGP-71 protein, candidate Avirulence gene effector vH6, interacts with its predicted target proteins in wheat, the TaSKP proteins.  E3 ligase mimicry is a well-characterized strategy of effector-based attack in certain bacterial plant pathogens. Our analyses suggest that like these bacteria, the HF may confound plant defenses and redirect plant cell development by overwhelming its host with E3 ligase mimics.  To our knowledge, these gene families constitute the first example of insect-derived E3 ligase mimics used as effector proteins.