pH Biosensing by PI4P Regulates Cargo Sorting at the TGN

Phosphoinositides, diacylglycerolpyrophosphate, ceramide-1-phosphate, and phosphatidic acid belong to a unique class of membrane signaling lipids that contain phosphomonoesters in their headgroups having pKa values in the physiological range. The phosphomonoester headgroup of phosphatidic acid enables this lipid to act as a pH biosensor as changes in its protonation state with intracellular pH regulate binding to effector proteins. Here, we demonstrate that binding of pleckstrin homology (PH) domains to phosphatidylinositol 4-phosphate (PI4P) in the yeast trans-Golgi network (TGN) is dependent on intracellular pH, indicating PI4P is a pH biosensor. pH biosensing by TGN PI4P in response to nutrient availability governs protein sorting at the TGN, likely by regulating sterol transfer to the TGN by Osh1, a member of the conserved oxysterol-binding protein (OSBP) family of lipid transfer proteins. Thus, pH biosensing by TGN PI4P allows for direct metabolic regulation of protein trafficking and cell growth

Analysis of the key elements of FFAT-like motifs identifies new proteins that potentially bind VAP on the ER, including two AKAPs and FAPP2.

Two phenylalanines (FF) in an acidic tract (FFAT)-motifs were originally described as having seven elements: an acidic flanking region followed by 6 residues (EFFDA-E). Such motifs are found in several lipid transfer protein (LTP) families, and they interact with a protein on the cytosolic face of the ER called vesicle-associated membrane protein-associated protein (VAP). Mutation of which causes ER stress and motor neuron disease, making it important to determine which proteins bind VAP. Among other proteins that bind VAP, some contain FFAT-like motifs that are missing one or more of the seven elements. Defining how much variation is tolerated in FFAT-like motifs is a preliminary step prior to the identification of the full range of VAP interactors

Liposome-Binding Assays to Assess Specificity and Affinity of Phospholipid–Protein Interactions

Protein-lipid interactions play an important role in cellular protein relocation, activation and signal transduction. The liposome-binding assay is a simple and inexpensive method to examine protein-lipid binding in vitro. The phospholipids used for liposome production are dried and hydrated. Subsequent extrusion of the phospholipid mixture ensures the production of large unilamellar vesicles (LUV) filled with raffinose. Those LUVs can be easily separated from the aqueous solution by centrifugation. By incubating a protein of interest with the LUVs and subsequent centrifugation steps, the bound protein fraction can be determined using Western Blot or Coomassie staining. This technique enables analysis of protein-lipid binding affinity and specificity

Rom-1 is required for rod photoreceptor viability and regulation of disk morphogenesis

The homologous membrane proteins Rom-1 and peripherin-2 are localized to the disk rims of photoreceptor outer segments (OSs), where they associate as tetramers and larger oligomers1, 2, 3. Disk rims are thought to be critical for disk morphogenesis, OS renewal4 and the maintenance of OS structure5, but the molecules which regulate these processes are unknown. Although peripherin-2 is known to be required for OS formation (because Prph2−/− mice do not form OSs; ref. 6), and mutations in RDS (the human homologue of Prph2) cause retinal degeneration7, the relationship of Rom-1 to these processes is uncertain. Here we show that Rom1−/− mice form OSs in which peripherin-2 homotetramers are localized to the disk rims, indicating that peripherin-2 alone is sufficient for both disk and OS morphogenesis. The disks produced in Rom1−/− mice were large, rod OSs were highly disorganized (a phenotype which largely normalized with age) and rod photoreceptors died slowly by apoptosis. Furthermore, the maximal photoresponse of Rom1−/− rod photoreceptors was lower than that of controls. We conclude that Rom-1 is required for the regulation of disk morphogenesis and the viability of mammalian rod photoreceptors, and that mutations in human ROM1 may cause recessive photoreceptor degeneration