Effector protein involved in gene-for-gene resistance in tomato plants. It is recognized by the host Pto resistance protein and elicits Pto and Prf-dependent hypersensitive response (HR) and programmed cell death (PCD), resulting in host immunity. In susceptible plants, acts as a virulence factor by suppressing PCD and HR-based plant immunity. This function requires its E3 ubiquitin ligase activity probably by recruiting E2 enzymes and transferring ubiquitin molecules to cellular proteins involved in regulation of PCD and targeting them for degradation. Also, induces expression of host genes involved in ethylene biosynthesis and signaling, in particular ACO1 and ACO2, encoding the ethylene-forming enzyme ACC oxidase. Interacts physically with plant cell Pto. Secreted via type III secretion system (TTSS). Localized to the plant cell cytoplasm. Transcriptionally induced by HrpL. Auto-ubiquitinated. Unlike many effector proteins, it is widely conserved among diverse genera of plant pathogens including Xanthomonas, Erwinia and many strains of Pseudomonas. Acts as a general eukaryotic PCD inhibitor in plants and yeast. Belongs to the HopAB family.