What pathogens have taught us about posttranslational modifications

Abstract

Pathogens use various mechanisms to manipulate host processes to promote infection. Decades of research on pathogens have revealed not only the molecular mechanisms that these microbes use to replicate and survive within host cells, but also seminal information on how host signaling machinery regulates cellular processes. Among these discoveries are mechanisms involving posttranslational modifications that alter the activity, localization, or interactions of the modified protein. Herein, we examine how pathogens have contributed to our basic understanding of three posttranslational modifications: phosphorylation, NMPylation, and ubiquitylation. Over the years, technologies, techniques and research tools have developed side by side with the study of pathogens, facilitating the discovery of protein modifications and furthering our understanding of how they contribute to both infection and cellular functions.

Figure 1. Pathogens antagonize host signaling

(A) Yersinia YopJ acetylates serine/threonine residues on the activation lop of MAPK kinases (MKK) to prevent activation by phosphorylation. (B) Shigella OspF uses β-eliminylation to remove the phosphate from a phosphorylated MAPK, resulting in an irreversibly inactive kinase. (C) Xhanthomonas campestris AvrAc uses UMPylation to modify MAPK kinases on serine/threonine residues within the activation loop of MAPK kinases (MKK) to prevent phosphorylation-mediated activation. (D) The family of Cif-like effectors deamidate ubiquitin, resulting in substrates that cannot be used for ubiquitin elongation.

Figure 2. Legionella uses AMPylation and phosphocholination to regulate the Rab1 GTPase. Rab1 is sequestered in the cytoplasm by a guanine dissociation inhibitor (GDI)

Upon infection, Rab1 is recruited to the Legionella-containing vacuole (LCV) membrane followed by activation by the Legionella GEF DrrA. It is then modified by either AMPylation or phosphocholination by the bacterial effectors DrrA or AnkX, respectively. The modified form of Rab1 cannot interact with host GEFs and GAPs. The PTMs on activated GTP-bound Rab1 can be reversed by deAMPylation or dephosphocholination by the effectors SidD or Lem3, respectively. The GTP-bound Rab1 can then be inactivated by the Legionella GAP LepB.

Figure 3. Different mechanisms and substrates used by E3 ubiquitin ligase families

(A) The HECT and IpaH families of E3 ligases accept ubiquitin from an E2 and then transfer it to a lysine residue on the substrate protein. (B) The RING E3 ligases do not accept ubiquitin from the E2, but rather facilitate the direct transfer of ubiquitin from the E2 to a lysine residue on the substrate. (C) The viral K3/K5 and the eukaryotic MARCH family of E3 ligases are membranal proteins that mediate the ubiquitylation of non-lysine residues on substrates, such as cysteine, serine, and threonine. They can also target membranal proteins by mediating non-canonical Lys63 and Lys11 linkages in polyubiquitin chains, as opposed to the canonical Lys48 linkages that are usually mediated by HECT and RING E3s.