Innate immunity kinase TAK1 phosphorylates Rab1 on a hotspot for posttranslational modifications by host and pathogen.

TGF-β activated kinase 1 (TAK1) is a critical signaling hub responsible for translating antigen binding signals to immune receptors for the activation of the AP-1 and NF-κB master transcriptional programs. Despite its importance， known substrates of TAK1 are limited to kinases of the MAPK and IKK families and include no direct effectors of biochemical processes. Here， we identify over 200 substrates of TAK1 using a chemical genetic kinase strategy. We validate phosphorylation of the dynamic switch II region of GTPase Rab1， a mediator of endoplasmic reticulum to Golgi vesicular transport， at T75 to be regulated by TAK1 in vivo. TAK1 preferentially phosphorylates the inactive (GDP-bound) state of Rab1. Phosphorylation of Rab1 disrupts interaction with GDP dissociation inhibitor 1 (GDI1)， but not guanine exchange factor (GEF) or GTPase-activating protein (GAP) enzymes， and is exclusive to membrane-localized Rab1， suggesting phosphorylation may stimulate Rab1 membrane association. Furthermore， we found phosphorylation of Rab1 at T75 to be essential for Rab1 function. Previous studies established that the pathogen Legionella pneumophila is capable of hijacking Rab1 function through posttranslational modifications of the switch II region. Here， we present evidence that Rab1 is regulated by the host in a similar fashion， and that the innate immunity kinase TAK1 and Legionella effectors compete to regulate Rab1 by switch II modifications during infection.