Highlights
•Binding of TOM1 VHS to ubiquitin is enhanced by an internal DXXLL motif
•Binding of TOM1 to PtdIns5P is maximal under acidic conditions
•TOLLIP’s binding to PtdIns5P also increases as pH acidifies
•A tight TOM1-TOLLIP complex binds to PtdIns5P under acidic conditions
Summary
Target of Myb1 (TOM1) facilitates the transport of endosomal ubiquitinated proteins destined for lysosomal degradation; however, the mechanisms regulating TOM1 during this process remain unknown. Here, we identified an adjacent DXXLL motif-containing region to the TOM1 VHS domain, which enhances its affinity for ubiquitin and can be modulated by phosphorylation. TOM1 is an endosomal phosphatidylinositol 5-phosphate (PtdIns5P) effector under Shigella flexneri infection. We pinpointed a consensus PtdIns5P-binding motif in the VHS domain. We show that PtdIns5P binding by TOM1 is pH-dependent, similarly observed in its binding partner TOLLIP. Under acidic conditions, TOM1 retained its complex formation with TOLLIP, but was unable to bind ubiquitin. S. flexneri infection inhibits pH-dependent endosomal maturation, leading to reduced protein degradation. We propose a model wherein pumping of H+ to the cytosolic side of endosomes contributes to the accumulation of TOM1, and possibly TOLLIP, at these sites, thereby promoting PtdIns5P- and pH-dependent signaling, facilitating bacterial survival….
