Met1‐linked ubiquitination in immune signalling

N ‐terminal methionine‐linked ubiquitin (Met1‐Ub), or linear ubiquitin, has emerged as a central post‐translational modification in innate immune signalling. The molecular machinery that assembles, senses and, more recently, disassembles Met1‐Ub has been identified, and technical advances have enabled the identification of physiological substrates for Met1‐Ub in response to activation of innate immune receptors. These discoveries have significantly advanced our understanding of how nondegradative ubiquitin modifications control proinflammatory responses mediated by nuclear factor‐κB and mitogen‐activated protein kinases. In this review, we discuss the current data on Met1‐Ub function and regulation, and point to some of the questions that still remain unanswered.