The SNARE protein SYP22 in Arabidopsis interacts with ATG8 to promote the autophagosome-vacuole fusion

In eukaryotic cells, excess or damaged cytoplasmic constituents are targeted into lytic compartments via autophagosomal membrane trafficking. Biogenesis of autophagosomes in fungi, metazoans, and plants relies on the conserved ATG (autophagy related) proteins. The machinery responsible for autophagosome turnover has been elucidated in yeast and metazoans, but not in plants. Here we examined 14 soluble N-ethylmaleimide-sensitive-factor attachment protein receptors (SNAREs) in Arabidopsis thaliana by autophagy marker and genetic analyses. We identified SYP22 (Syntaxin of Plants 22) as a SNARE that is necessary for the efficient fusion of autophagosomes with the vacuole. Genetic disruption of SYP22 led to a reduction in autophagic flux and the accumulation of autophagosomes. The vacuolar Qa-SNARE SYP22 interacted with autophagosomal proteins, such as ATG8 and the R-SNARE VAMP724. Overall, our molecular and genetic analyses of Arabidopsis SNAREs underscore the importance of autophagosome-vacuole fusion in autophagic flux, and provide an insight into how plant vacuolar SNARE proteins recognize the autophagosome and mediate its fusion. As a unique mutant defective in the turnover of autophagosomes, syp22 will be useful for overcoming bottlenecks in plant autophagy research.
Abbreviations: AIM: Atg8-family interacting motif; ATG: autophagy related; BiFC: bimolecular fluorescence complementation; co-IP: co-immunoprecipitation; ConA: concanamycin A; DMSO: dimethyl sulfoxide; ER: endoplasmic reticulum; HOPS: homotypic fusion and protein sorting; LE: late endosome; PM: plasma membrane; PVC: prevacuolar compartment; SNARE: soluble N-ethylmaleimide-sensitive-factor attachment protein receptor; SYP: Syntaxin of Plants; TEM: transmission electron microscopy; TGN: trans-Golgi network; WT: wild type.