PROTEOLYSIS A Short Cut to Signaling

Urban and Freeman investigated the mechanisms whereby the intramembrane protease Rhomboid-1 achieves substrate specificity and defined a motif that could be used to identify rhomboid substrates. Rhomboid intramembrane proteases represent a widespread group of enzymes whose functions and substrates remain, for the most part, undefined. In Drosophila, Rhomboid-1 cleaves the protein Spitz in the Golgi apparatus, allowing Spitz to be secreted and to act as a ligand for the epidermal growth factor receptor (EGFR). AarA, another rhomboid membrane protease, has been implicated in activating intercellular signals in bacteria. Urban and Freeman coexpressed Rhomboid-1 with various proteins in COS cells and found that it showed specificity in cleaving Spitz but not the closely related transforming growth factor (TGF-), the EGFR receptor, or the Notch ligand Delta. The authors constructed chimeric proteins, in which various regions of Spitz and TGF- were systematically exchanged, and defined a small motif that lay entirely within the Spitz transmembrane domain that was both necessary and sufficient for substrate cleavage. Mutational analysis indicated that helix-disrupting residues--together with constraints on the extent of hydrophobicity--were critical for Rhomboid-1 function. Rhomboid proteins from bacteria and from vertebrates recognized the same motif, suggesting a common targeting strategy. The authors searched for other proteins cleaved within their transmembrane domains that had Spitz-like motifs and identified a family of proteins involved in parasitic invasion as rhomboid substrates. The cleavage motif can thus be used to identify new rhomboid substrates and thereby to better understand the functions of this widespread enzyme family.

S. Urban, M. Freeman, Substrate specificity of rhomboid intramembrane proteases is governed by helix-breaking residues in the substrate transmembrane region. Mol. Cell 11, 1425-1434 (2003). [Online Journal]