The Arabidopsis autophagy cargo receptors ATI1 and ATI2 bind to ATG8 via intrinsically disordered regions and are post-translationally modified upon ATG8 interaction
Selective autophagy has emerged as an important mechanism by which eukaryotic cells control the abundance of specific proteins. This mechanism relies on cargo recruitment to autophagosomes by receptors that bind to both the ubiquitin-like AUTOPHAGY8 (ATG8) protein through ATG8 interacting motifs (AIMs) and to the cargo to be degraded. In plants, two autophagy cargo receptors, ATG8 Interacting Protein 1 (ATI1) and 2 (ATI2), were identified early on, but their molecular properties remain poorly understood. Here, we show that ATI1 and ATI2 are transmembrane proteins with long N-terminal intrinsically disordered regions (IDRs). The N-terminal IDRs contain the functional AIMs, and we use nuclear magnetic resonance spectroscopy to directly observe the disorder-order transition of the AIM upon ATG8 binding. Our analyses also show that the IDRs of ATI1 and ATI2 are not equivalent, because ATI2 has properties of a fully disordered polypeptide, while ATI1 has properties consistent with a collapsed pre-molten globule-like conformation Interestingly, wild type ATI1 and ATI2 exist as distinct post-translationally modified forms. Specifically, different forms are detectable upon mutation of the AIM, suggesting that interaction of ATI1 and ATI2 to ATG8 is coupled to a change in their post-translational modification.