Palmitoylation of Hedgehog proteins

Abstract

Hedgehog (Hh) proteins are secreted signaling proteins that contain amide-linked palmitate at the N-terminus and cholesterol at the C-terminus. Palmitoylation of Hh proteins is critical for effective long- and short-range signaling. The palmitoylation reaction occurs during transit of Hh through the secretory pathway, most likely in the lumen of the ER. Attachment of palmitate to Hh proteins is independent of cholesterol modification and autoprocessing and is catalyzed by Hhat (Hedgehog acyltransferase). Hhat is a member of the membrane bound O-acyltransferase (MBOAT) family, a subgroup of multipass membrane proteins that catalyze transfer of fatty acyl groups to lipids and proteins. Several classes of secreted proteins have recently been shown to be substrates for MBOAT acyltransferases, including Hh proteins and Spitz (palmitoylated by Hhat), Wg/Wnt proteins (modified with palmitate and/or palmitoleate by Porcupine) and ghrelin (octanoylated by ghrelin O-acyltransferase). These findings highlight protein fatty acylation as a mechanism that not only influences membrane binding of intracellular proteins but also regulates the signaling range and efficacy of secreted proteins.

Figure 10.1

Multiple processing events generate the mature Shh signaling protein. Signal peptidase removes the N-terminal signal sequence. The autoprocessing domain mediates autocleavage and attachment of cholesterol to the C-terminus of the Shh signaling domain, while Hhat catalyzes attachment of palmitate via amide linkage to the N-terminus of the Shh signaling domain.

Figure 10.2

Potential mechanisms for Hhat-mediated N-Palmitoylation of Hedgehog proteins. (A) Formation of a thioester intermediate (1) is followed by an S-to-N intramolecular shift and generation of the amide-linked, palmitoylated hedgehog protein (2). (B) Alternatively, Hhat could catalyze direct attachment of palmitate via amide bond to the amine group of the N-terminal cysteine.