Abstract
Lipid transfer proteins acquire and release their lipid cargoes by interacting transiently with source and destination biomembranes. In the GlycoLipid Transfer Protein (GLTP) superfamily, the two-layer all-α-helical GLTP-fold defines proteins that specifically target sphingolipids (SLs) containing either sugar or phosphate headgroups via their conserved but evolutionarily-modified SL recognitions centers. Despite comprehensive structural insights provided by X-ray crystallography, the conformational dynamics associated with membrane interaction and SL uptake/release by GLTP superfamily members have remained unknown. Herein, we report insights gained from molecular dynamics (MD) simulations into the conformational dynamics that enable ceramide-1-phosphate transfer proteins (CPTPs) to acquire and deliver ceramide-1-phosphate (C1P) during interaction with 1-palmitoyl-2-oleoyl phosphatidylcholine bilayers. The focus on CPTP reflects this protein’s involvement in regulating pro-inflammatory eicosanoid production and autophagy-dependent inflammasome assembly that drives interleukin (IL-1β and IL-18) production and release by surveillance cells. We found that membrane penetration by CPTP involved α-6 helix and the α-2 helix N-terminal region, was confined to one bilayer leaflet, and was relatively shallow. Large-scale dynamic conformational changes were minimal for CPTP during membrane interaction or C1P uptake except for the α-3/α-4 helices connecting loop, which is located near the membrane interface and interacts with certain phosphoinositide headgroups. Apart from functioning as a shallow membrane-docking element, α-6 helix was found to adeptly reorient membrane lipids to help guide C1P hydrocarbon chain insertion into the interior hydrophobic pocket of the SL binding site.These findings support a proposed ‘hydrocarbon chain-first’ mechanism for C1P uptake, in contrast to the ‘lipid polar headgroup-first’ uptake used by most lipid-transfer proteins.
Highlights
Supplementary key words lipid-transfer proteins membranes/physical chemistry membranes/model protein structure human glycolipid transfer protein (GLTP) superfamily Arabidopsis accelerated cell death (ACD11) protein peripheral membrane penetration depth ceramide-1phosphate binding GlycoLipid Transfer Protein (GLTP)-fold alpha-helix induced lipid reorientation
In ceramide-1phosphate transfer protein (CPTP), the same general surface region has been implicated in membrane interaction as that of GLTP, but other details remain unclear [5, 10]
OPM/Positions of Proteins in Membranes (PPM) modeling relies on an implicit solvation model for the lipid bilayer to approximate protein positioning and calculate membrane penetration [26,27,28] but does not provide insights into the dynamic changes experienced by CPTP during membrane-lipid interaction or lipid-cargo binding
Summary
Like FAPP2, ceramide transfer protein relies an N-terminal PH domain to target phosphatidylinositol 4-phosphate and help guide interaction with specific membranes but uses a C-terminal StAR-related lipid transfer domain to bind ceramide during the back and forth shuttling needed to redistribute ceramides between closely apposed membranes [18, 19]. The issue of what guides FAPP2 to specific membranes is reasonably well established, only recently have insights begun to emerge for small, single-domain GLTP superfamily members. Owing to the emerging roles of C1P-transferring GLTP superfamily members in pathophysiological situations involving inflammation and autophagy, a need exists to better understand the functional dynamics that enable membrane interaction, SL uptake/release, and the atypicalbound lipid orientation associated with the GLTP protein superfamily.
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