Fujita et al. describe how the ubiquitination of host endosomal proteins helps capture invading bacteria inside autophagosomes.
In starving cells, double-membraned structures called autophagosomes engulf cytoplasmic content and deliver it to the lysosome for degradation and recycling. But autophagosomes can sometimes be more selective. In a process called xenophagy, for example, autophagosomes specifically form around invading bacteria. The engulfed pathogens are coated with ubiquitin, but which proteins are ubiquitinated, and how this aids bacterial capture, is unknown. One possibility is that proteins on the bacterial surface are ubiquitinated and then recognized by specific adaptor proteins, like p62, that link them to the autophagosomal protein LC3.
Using Salmonella or bacteria-mimicking beads, Fujita et al. found that, in fact, host endosomal proteins are ubiquitinated when xenophagy targets enter the cell and rupture the endosomal membrane. This ubiquitination was required to recruit several key autophagy proteins to bacteria-containing endosomes, including the ULK1 complex, which initiates autophagosome assembly, and the Atg16L1 complex, which helps conjugate LC3 to membrane lipids. These autophagy proteins were all recruited to damaged endosomes before LC3.
Atg16L1 bound directly to ubiquitin through its C-terminal WD domain. But Atg16L1 mutants lacking this domain were recruited to Salmonella-containing endosomes through two different backup mechanisms, including a direct interaction with the ULK1 subunit FIP200. Abolishing Atg16L1’s localization was sufficient to prevent LC3’s recruitment to ubiquitinated endosomes.
The researchers now want to investigate how cells recognize ruptured endosomal membranes and which host proteins are then ubiquitinated. Because the putative ubiquitin ligase targets host proteins, it can likely respond to a range of pathogens besides Salmonella.
Text by Ben Short