The Autophagy Receptor TAX1BP1 and the Molecular Motor Myosin VI Are Required for Clearance of Salmonella Typhimurium by Autophagy

Abstract

Autophagy plays a key role during Salmonella infection, by eliminating these pathogens following escape into the cytosol. In this process, selective autophagy receptors, including the myosin VI adaptor proteins optineurin and NDP52, have been shown to recognize cytosolic pathogens. Here, we demonstrate that myosin VI and TAX1BP1 are recruited to ubiquitylated Salmonella and play a key role in xenophagy. The absence of TAX1BP1 causes an accumulation of ubiquitin-positive Salmonella, whereas loss of myosin VI leads to an increase in ubiquitylated and LC3-positive bacteria. Our structural studies demonstrate that the ubiquitin-binding site of TAX1BP1 overlaps with the myosin VI binding site and point mutations in the TAX1BP1 zinc finger domains that affect ubiquitin binding also ablate binding to myosin VI. This mutually exclusive binding and the association of TAX1BP1 with LC3 on the outer limiting membrane of autophagosomes may suggest a molecular mechanism for recruitment of this motor to autophagosomes. The predominant role of TAX1BP1, a paralogue of NDP52, in xenophagy is supported by our evolutionary analysis, which demonstrates that functionally intact NDP52 is missing in Xenopus and mice, whereas TAX1BP1 is expressed in all vertebrates analysed. In summary, this work highlights the importance of TAX1BP1 as a novel autophagy receptor in myosin VI-mediated xenophagy. Our study identifies essential new machinery for the autophagy-dependent clearance of Salmonella typhimurium and suggests modulation of myosin VI motor activity as a potential therapeutic target in cellular immunity.

Dynamic recruitment and activation of ALS-associated TBK1 with its target optineurin are required for efficient mitophagy

“…Studies on xenophagy have also identified a key regulatory role for TBK1, a serine/threonine kinase that has been demonstrated to interact with OPTN, NDP52, and TAX1BP1 (16,24,25). Here, we found that OPTN and TBK1 translocate to depolarized mitochondria with similar kinetics.…”

“…Along with OPTN, NDP52 and TAX1BP1 have been shown to function in the autophagic clearance of invasive bacteria (16,24,25), and, more recently, these adaptors have been implicated in the long-term clearance of depolarized mitochondria (13,14). However, the relative dynamics with which OPTN, NDP52, and TAX1BP1 are recruited to depolarized mitochondria have not been examined.…”

Intracellular Salmonella induces aggrephagy of host endomembranes in persistent infections

“…28 Autophagy machinery is recruited to these sites by CALCOCO2, SQSTM1, OPTN, and CALCOCO3, the latter a CALCOCO2 paralog. 23,30,31 In fibroblasts, we were unable to detect ubiquitin in the LGP C aggregate. Moreover, while in epithelial cells S. Typhimurium uses deubiquitinases like SseL to limit selective autophagy against ALIS, 59 our study in fibroblasts shows that the pathogen translocates SseL but that this SPI2 effector is not responsible for the lack of ubiquitin in the LGP C aggregate.…”

“…According to our FRAP data, we favor the idea of recognition by the autophagy machinery based on the insoluble state of the LGP C aggregate, as it was proposed for proteinaceous inclusions. 54 Future studies on other autophagy components very recently associated with S. Typhimurium clearance, such as MYO6 (myosin VI) and the autophagy receptor CALCOCO3, 31 might also shed light into the mechanism responsible for the aggrephagy event described here.…”

“…28 Ubiquitinated components on the pathogen surface and LGALS8 act as danger sensors recognized by diverse autophagy receptors, including CALCOCO2/NDP52, SQSTM1/p62, OPTN/optineurin and CALCOCO3/TAX1BP1. 23,[29][30][31] Diacylglycerol (DAG) signaling can also target S. Typhimurium to xenophagy. 32 These xenophagy events occur at early stages of the infection, 1 to 2 h after bacterial invasion of the epithelial cell.…”