All WNT proteins except Drosophila WntD are lipid modified. Lipid modifications contribute to the hydrophobicity and poor solubility of all known WNT ligands with the exception of Drosophila WntD. Acylation is required for their secretion from the cell and their ability to bind to FRZ receptors (reviewed in MacDonald et al, 2009; Takada et al, 2006; Janda et al, 2012; Herr and Basler, 2012; Ching et al, 2008). Although an initial study suggested that conserved Cys77 in mouse Wnt3a was palmitoylated (Willert et al, 2003), further work showed that mutation of this residue had minimal effect on WNT secretion (Komekado et al, 2007). In contrast, addition of palmitoleic acid to mWnt3a Ser209 is essential for WNT secretion, and mutant S209A is largely retained in the ER (Takada et al, 2006; Galli and Burrus, 2011). This serine residue is conserved at this position in all known WNTs with the exception of Drosophila WntD (Ching et al, 2008; Herr and Basler, 2012). A recent crystal structure of Xenopus WNT8 in complex with a Frizzled cysteine-rich-domain shows a single lipid modification on the conserved serine residue, while the conserved cysteine participates in a disulphide bond (Janda et al, 2012). In addition to being required for secretion, the lipid at S209 also makes direct contact with a groove in the Frizzled receptor and is thus essential for binding (Janda et al, 2012).
Porcupine is a conserved multi-pass transmembrane ER protein that has an O-acyl-transferase domain (van den Heuvel et al, 1993; Kadowaki et al, 1996; Hofmann, 2000). First identified in Drosophila, Porcupine is a WNT-specific modulator that is required for Wingless processing and secretion (Kadowaki et al, 1996). In porcn-deficient cells, Wg and WNT3A have decreased palmitoylation at S209 and accumulate in the ER (Takada et al, 2006), and mutations in PORCN eliminate all WNT signalling and cause embryonic lethality in mice (Barrott et al, 2011; Biechele et al, 2011). Recent studies show that PORCN is required for activity of all human WNT ligands (Proffitt et al, 2012; Najdi et al, 2012).