The complex life of rhomboid pseudoproteases

Rhomboid pseudoproteases are catalytically inactive members of the rhomboid superfamily. The founding members, rhomboids, were first identified in Drosophila as serine intramembrane proteases that cleave transmembrane proteins, enabling signaling. This led to the discovery of the wider rhomboid superfamily, a clan that in metazoans is dominated by pseudoproteases. These so-called rhomboid pseudoproteases inherited from their catalytically active ancestors a conserved rhomboid-like domain and a propensity to regulate signaling. Lacking catalytic activity, they developed new 'pseudoenzyme' functions that include regulating the trafficking, turnover, and activity of their client proteins. Rhomboid pseudoproteases have preeminent roles in orchestrating immune cell activation, antiviral responses, and cytokine release in response to microbial infection, or in chronic diseases, and have also been implicated in growth factor signaling, cancer, and, more recently, metabolism. Here, we discuss the mechanism(s) of action of rhomboid pseudoproteases, contrasted with rhomboid proteases. We also highlight the roles of rhomboid pseudoproteases in mammalian physiology, which, quite paradoxically among pseudoenzymes, is understood much better than active rhomboids.