Lipid News

Transport of O-palmitoleated Wnts: Where does the lipid go?

Rie Nygaard Filippo Mancia
By Rie Nygaard and Filippo Mancia
Feb. 16, 2021

Wnts are evolutionarily conserved ligands that signal at short range to regulate morphogenesis, cell fate and stem cell renewal. The first and essential step in Wnts' secretion is their O-palmitoleation by the enzyme porcupine, or PORCN. This lipid modification is unique to Wnts and crucial for limiting their diffusion, restricting Wnt signaling to short range.

Lipid-news-445x561.jpg
Rie Nygaard & Filippo Mancia
The structure of human O-palmitoleated WNT8A (green) in complex with its dedicated carrier WLS (blue). The O-palmitoleate is shown as yellow spheres, and the main structural elements are labeled.

Modification of Wnts has been shown to be essential to their signaling capabilities. After their O-palmitoleation in the endoplasmic reticulum, or ER, Wnts are loaded onto their dedicated transporter Wntless, or WLS, an integral membrane protein with a small soluble domain in the ER lumen. O-palmitoleated Wnts associated with WLS then travel to the plasma membrane, where they are transferred to receptors, such as Frizzled, on the membranes of target cells, in turn triggering the activation of signaling pathways. Structures of Wnt in complex with the cysteine-rich domain, or CRD, of Frizzled have shown how the lipid modification of Wnt is central to binding, with the O-palmitoleate buried deep in a hydrophobic groove of the CRD.

Questions remained about Wnts' transfer from the ER to target cells and the role of the O-palmitoleate: How does the transfer of the O-palmitoleated Wnt from PORCN to WLS occur? Where does the O-palmitoleate reside when Wnts are in complex with WLS, within the protein or in the membrane? How are Wnts released from WLS, and how do they travel with their water-insoluble cargo to neighboring cells?

We recently reported the structure of human O-palmitoleated WNT8A in complex with WLS, determined by single-particle cryo-electron microscopy to 3.2 Å resolution. The structure shows that the WLS membrane domain has close structural homology to G protein–coupled receptors, or GPCRs, with the addition of a transmembrane helix connecting the N-terminus to its luminal domain.

Based on the structures of Wnt bound to the CRD domain of Frizzled, we expected that the O-palmitoleate would be inserted into a hydrophobic binding site within the luminal domain of WLS. Instead, the Wnt hairpin carrying the lipid as a covalent attachment inserts deeply into a conserved hydrophobic cavity in the GPCR-like domain with the O-palmitoleate protruding between two transmembrane helices into the lipid bilayer.

We observed an extensive binding surface between Wnt and WLS, consistent with the known tight interaction between the two, while the energetically favorable environment of the membrane sheltered the hydrophobic cargo. A large opening to the bilayer within the membrane domain of WLS might be the route for shuttling the O-palmitoleate from PORCN to WLS, maintaining the lipid within the bilayer during the transfer and possibly involving a direct interaction between the enzyme and the carrier.

Comparing the structure of Wnt in complex with WLS to the structures of Wnt bound to the CRD domain of Frizzled, we observed a large conformational change on a separate Wnt hairpin, which may be important for its one-way transfer to receiving cells. For this transfer to occur, the O-palmitoleate must be extracted from the lipid bilayer and transferred to the CRD domain of Frizzled.

It's unclear if and when this involves other proteins and whether the transfer occurs in cis (the same) or trans (the opposite) membranes.

This work provides molecular-level insights into a central mechanism in animal body plan development and stem cell biology. We believe it opens up a new direction to explore membrane protein–lipid interactions.

Rie Nygaard
Rie Nygaard

Rie Nygaard is an associate research scientist in Filippo Mancia’s lab in the department of physiology and cellular biophysics at Columbia University Irving Medical Center.

Filippo Mancia
Filippo Mancia

Filippo Mancia is an associate professor and co-director of graduate education in the department of physiology and cellular biophysics at Columbia University Irving Medical Center.

Join the ASBMB Today mailing list

Sign up to get updates on articles, interviews and events.

Latest in Science

Science highlights or most popular articles

Ceramides’ role in liver disease
Lipid News

Ceramides’ role in liver disease

May 5, 2021

These biologically active sphingolipids have roles in apoptosis, inflammation and insulin resistance, all critical factors in the pathogenesis of chronic liver disease.

Winners of the ‘aha moments’ essay contest
Contest

Winners of the ‘aha moments’ essay contest

May 4, 2021

To celebrate our three journals going open access, we invited readers to share their moments of discovery in science. Here are the first, second and third place winners.

The 17th-century cloth merchant who discovered the vast realm of tiny microbes
News

The 17th-century cloth merchant who discovered the vast realm of tiny microbes

May 2, 2021

Although untrained in science, Antonie van Leeuwenhoek became the greatest lens-maker of his day, discovered microscopic life forms and is known today as the “father of microbiology.”

Do kids really need to be vaccinated for COVID?
News

Do kids really need to be vaccinated for COVID?

May 1, 2021

Many experts argue that Covid-19 cannot be curbed without vaccinating children. But others aren’t so sure.

Targeting nitrated proteins could lead to new cancer drugs
Annual Meeting

Targeting nitrated proteins could lead to new cancer drugs

April 30, 2021

Researchers are studying these proteins’ potential as markers that could make tumor cells easy targets for new therapies.

Gene changes and long-haul COVID
Annual Meeting

Gene changes and long-haul COVID

April 30, 2021

Airway cells exposed to SARS-CoV-2 spike protein exhibited persisting changes in gene expression.