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Fig. 3. Post-translational processing of secreted palmitoylated proteins. (A) Hedgehog is synthesized on membrane-bound ribosomes and translocated into the lumen of the ER by a signal-peptide-mediated process (orange). The hedgehog precursor undergoes proteolytic processing events to remove the signal peptide and a second autocatalytic cleavage generating the N-terminal signaling domain (blue) and a C-terminal autoprocessing domain (pink). Following cleavage, cholesterol is added at the C-terminus of the signaling domain. The N-terminus is palmitoylated, presumably by the Rasp PAT. Modification with cholesterol is coupled with autocatalytic cleavage, but is independent of palmitate addition. The order of addition of the two lipids is unknown. Although shown as occurring in the ER, the intracellular compartment where autocatalytic cleavage and lipidation of Hedgehog occurs is unknown. In the predicted structure of the unmodified signaling domain of Hedgehog, both lipid modifications emerge from the same face of the molecule, presumably tethering the protein to the membrane (Ho and Scott, 2002). The lipid modifications might facilitate targeting of Hedgehog to lipid rafts on the cell surface (Rietveld et al., 1999). However, Hedgehog is secreted in a hexameric form (Zeng et al., 2001), raising the possibility that the lipid modifications might form a hydrophobic core (Ho and Scott, 2002). (B) The Wingless precursor also enters the ER lumen via a signal-peptide-mediated translocation followed by signal peptide cleavage (orange). In the model described in the text, Wingless is palmitoylated by Por (purple) on a conserved cysteine residue prior to glycosylation. Rasp and Por are integral membrane proteins, but their topology is not known.
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