Arabidopsis Accelerated Cell Death 11, ACD11, Is a Ceramide-1-Phosphate Transfer Protein and Intermediary Regulator of Phytoceramide Levels

Abstract

SUMMARY The accelerated-cell-death11 (acd11) mutant of Arabidopsis provides a genetic model for studying immune response activation and localized cellular suicide that halts pathogen spread during infection in plants. Here, we elucidate ACD11 structure/function and show that acd11 disruption dramatically alters the in vivo balance of sphingolipid mediators that regulate eukaryotic programmed cell death. In acd11 mutants, normally low ceramide-1-phosphate (C1P) levels become elevated, but the relatively abundant cell death inducer, phytoceramide, rises acutely. ACD11 exhibits selective intermembrane transfer of C1P and phyto-C1P. Crystal structures establish C1P binding via a surface-localized, phosphate headgroup recognition center connected to an interior hydrophobic pocket that adaptively ensheaths lipid chains via a cleft-like gating mechanism. Point mutation mapping confirms functional involvement of binding-site residues. A π-helix (π-bulge) near the lipid-binding cleft distinguishes apo-ACD11 from other GLTP-folds. The global two-layer, α-helically-dominated, ‘sandwich’ topology displaying C1P-selective binding identifies ACD11 as the plant prototype of a new GLTP-fold subfamily.

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

“…Arabidopsis thaliana contains a CPTP orthologue known as ACD11 that functions as a lipid transfer protein for C1P and phyto-C1P (13). The ACD11 name originates from the accelerated cell death (acd) phenotype observed upon disruption of the acd11 gene (16).…”

“…The ACD11 name originates from the accelerated cell death (acd) phenotype observed upon disruption of the acd11 gene (16). In the Arabidopsis acd11 mutant, altered sphingolipid homeostasis manifests as moderately increased C1P and highly increased ceramide that help drive a programmed cell death response (13). Arabidopsis acd mutants provide genetic models for studying localized cell suicide that can halt the spread of pathogen infection and immune response activation in plants (2,17,18).…”

“…CPTPs, like GLTPs, occur almost ubiquitously in eukaryotes but play key roles in controlling inflammation and programmed cell death processes (12)(13)(14). Arabidopsis thaliana contains a CPTP orthologue known as ACD11 that functions as a lipid transfer protein for C1P and phyto-C1P (13).…”

“…The GLTP-fold consists of multiple ␣-helices arranged in a two-layer "sandwich" topology to form a single sphingolipid binding site (8 -12). A newly discovered family within the GLTP superfamily uses a modified GLTP-fold to selectively transfer ceramide 1-phosphate (C1P) rather than glycosphingolipids between membranes (12)(13)(14). Human C1P transfer protein (CPTP; 214 amino acids) is encoded by a threeexon transcript from the single-copy CPTP gene 5 on chromosome 1 (locus 1p36.33).…”

ORM Expression Alters Sphingolipid Homeostasis and Differentially Affects Ceramide Synthase Activities

“…In addition, sphingolipids, particularly those with very-long-chain fatty acids (VLCFAs), are integrally associated with Golgi-mediated protein trafficking that underlies processes related to the growth of plant cells (Bach et al, 2008(Bach et al, , 2011Markham et al, 2011;Melser et al, 2011). Furthermore, sphingolipids function through their bioactive long-chain base (LCB) and ceramide metabolites to initiate programmed cell death (PCD), important for mediating plant pathogen resistance through the hypersensitive response (Greenberg et al, 2000;Liang et al, 2003;Shi et al, 2007;Bi et al, 2014;Simanshu et al, 2014).…”