The Golgi protein FAPP2 tubulates membranes by a hydrophobic wedge in its PH domain

Abstract

Outgoing traffic from the Golgi complex diverges into different directions. In epithelial cells one major route is to the apical membrane. The machinery responsible for the formation of apical carriers is poorly understood. Several proteins have been identified that play a role. One such protein is the Golgi-associated four-phosphate adaptor protein 2 (FAPP2). FAPP2 is a cytosolic protein, consisting of: a N-terminal pleckstrin homology (PH) domain recognizing the Golgi marker, phosphatidylinositol 4-phosphate [PI(4)P], followed by a central proline-rich region, and a glycolipid transfer protein (GLTP)-like domain towards the C-terminus. Recent studies have shown that FAPP2 has transfer activity for glucosylceramide (GlcCer) both in vitro and in cells, but so far a detailed view at the protein from the structural and functional perspective was missing.

We discovered that purified FAPP2 tubulates lipid membranes in a PI(4)P-dependent fashion. This finding would fit well with properties previously attributed to the protein and adds an important missing function in the machinery responsible for apical transport. Additionally, applying analytical ultracentrifugation and small angle X-ray scattering we show that FAPP2 is a dimeric protein in solution, having a curved banana-like shape of 30 nm length. Based on the NMR structure of the homologous FAPP1-PH domain and lipid micelle docking experiments, we can show that the PH domain contains an exposed hydrophobic wedge that penetrates into the membrane bilayer, which when mutated inactivated the tubulation capacity. It is likely that this design principle for the FAPP-PH domain can be extended as a feature to a subset of other PH domain containing proteins including the ceramide transfer protein (CERT) and a number of oxysterol binding proteins (OSBP), all of which share the conserved wedge sequence.

Speaker

Dr. Ünal Coskun, Max-Planck Institute of Molecular Cell Biology and Genetics, Germany.