Cell Biology

Pseudoproteases as Inhibitors

Science Signaling  05 Apr 2011:
Vol. 4, Issue 167, pp. ec95
DOI: 10.1126/scisignal.4167ec95

In Drosophila melanogaster, members of the Rhomboid family of serine proteases cleave epidermal growth factor (EGF)–like membrane proteins, thus releasing the ligand portion of the proteins into the extracellular space where they can bind to EGF receptors (EGFRs). An evolutionarily conserved group of rhomboid-like proteins known as inactive rhomboids (iRhoms) are predicted to lack catalytic activity, but their functions have been elusive. Zettl et al. confirmed that Drosophila iRhom, human iRhom1, and mouse iRhom2 were catalytically inactive. The mRNA encoding iRhom was detected mainly in neural tissues in Drosophila embryos and exogenously expressed Drosophila iRhom, human iRhom1, or mouse iRhom2 localized to the endoplasmic reticulum (ER) in transfected COS7 cells. Drosophila lacking iRhom showed reduced daytime activity caused by decreased wakefulness compared with wild-type flies, a phenotype that was rescued by neural expression of iRhom. Drosophila with neural activation of EGFR signaling driven by transgenic expression of Rhomboid-1 and Star (which promotes the availability of some rhomboid substrates) show a similar phenotype, suggesting that iRhom inhibited EGFR signaling. Various genetic manipulations in the developing imaginal disc confirmed that iRhom blocked EGFR signaling. In transfected COS7 cells, coexpression of Drosophila iRhom prevented Rhomboid-1–induced release of the Drosophila EGF family ligand Gurken. Similarly, expression of mouse iRhom2 inhibited release of soluble mouse EGF as induced by active mouse rhomboid (RHBDL2). iRhoms decreased the abundance of several EGF family ligands in COS7 cells, which lack endogenous rhomboid activity, thus indicating that iRhoms did not act on rhomboids. The iRhom-induced reduction in the abundance of mouse EGF was attenuated by proteasomal inhibitors. Combined with the ER localization of iRhoms, this finding suggested that iRhoms promoted ER-associated degradation (ERAD) of EGF family ligands. Indeed, RNA interference in Drosophila directed against the ERAD factors Hrd1 or EDEM2 relieved the inhibition of EGFR signaling caused by overexpression of the EGFR inhibitors Sprouty or Argos. Thus, iRhoms attenuate EGFR signaling by promoting ERAD of EGF family ligands, a function that appears to be evolutionarily conserved. The authors propose that other pseudoenzymes may also have evolved to oppose the actions of the enzymes from which they were derived.

M. Zettl, C. Adrain, K. Strisovsky, V. Lastun, M. Freeman, Rhomboid family pseudoproteases use the ER quality control machinery to regulate intercellular signaling. Cell 145, 79–91 (2011). [PubMed]