Structural features of Hh proteins. (a) Signal peptide sequence for protein export (SS, yellow), the amino-terminal signaling domain (HhN, green), and the autocatalytic carboxy-terminal domain (HhC, black) are indicated. Both HhN and HhC domains are also found in proteins other than the Hh family, and are therefore globally referred to as 'Hedge' and 'Hog', respectively. The Hog domain itself can be separated into two regions; the first two-thirds shares similarity with self-splicing inteins, and this module has been named Hint, whereas the carboxy-terminal third binds cholesterol in Hh proteins and has been named the sterol-recognition region (SRR) . In Hog proteins other than Hh, that is, Hh-related proteins, this region is referred to as ARR (adduct recognition region) , as the nature of the adduct is not known. (b) Intramolecular autoprocessing of Hh. Acids and bases assisting in catalysis are not shown (figure adapted from [14, 70]).