Phosphatidylserine Stimulates Ceramide 1-Phosphate (C1P) Intermembrane Transfer by C1P Transfer Proteins

Area of Science:

• Plant pathology and immunology
• Lipid biochemistry and signaling

Background:

• Localized cell suicide in plants, exemplified by the Arabidopsis accelerated-cell-death 11 (acd11) mutant, is crucial for halting pathogen spread and modulating immune responses.
• The ACD11 protein, a lipid transfer protein specific for ceramide 1-phosphate (C1P) and phyto-C1P, is essential for sphingolipid homeostasis; its depletion disrupts plant immunity.
• The C1P binding sites in ACD11 and human ceramide-1-phosphate transfer protein (CPTP) are characterized by surrounding cationic residues, suggesting potential interactions with anionic lipids.

Purpose of the Study:

• To investigate the functional regulation of ACD11 and CPTP by anionic phosphoglycerides.
• To elucidate the role of specific phosphoglyceride headgroups in modulating C1P intermembrane transfer mediated by these lipid transfer proteins.

Main Methods:

• Utilized in vitro assays to measure ceramide 1-phosphate (C1P) intermembrane transfer.
• Incorporated various anionic phosphoglycerides, including phosphatidic acid, phosphatidylglycerol, and phosphatidylserine, into C1P source vesicles at varying concentrations.
• Compared the effects of these anionic phosphoglycerides on C1P transfer by ACD11 and CPTP with their effect on human glycolipid lipid transfer protein (GLTP).

Main Results:

• Anionic phosphoglycerides, specifically 1-palmitoyl-2-oleoyl-phosphatidic acid and 1-palmitoyl-2-oleoyl-phosphatidylglycerol (at ≤15 mol %), inhibited C1P transfer by ACD11 and CPTP.
• Conversely, 1-palmitoyl-2-oleoyl-phosphatidylserine significantly stimulated C1P transfer by ACD11 and CPTP, indicating a headgroup-specific regulatory mechanism.
• Soluble phosphatidylserine did not stimulate transfer, and none of the tested anionic phosphoglycerides affected C1P transfer by the glycolipid-specific GLTP, which lacks extensive cationic residues near its binding site.

Conclusions:

• These findings provide the first evidence for phosphoglyceride headgroup-specific regulatory sites on the surface of lipid transfer proteins belonging to the GLTP superfamily.
• Delineates novel differences between C1P-specific (ACD11, CPTP) and glycolipid-specific (GLTP) members of the GLTP superfamily regarding lipid transfer regulation.
• Suggests a mechanism where specific anionic phosphoglycerides, particularly phosphatidylserine, play a regulatory role in C1P-mediated sphingolipid homeostasis and potentially plant immune signaling.