Biology subjects

Jaxel, C.

Publications and source records attributed to Jaxel, C..

3 recordsLinked to original sources

bioRxiv2025

Ist2, a protein involved in phosphatidylserine transport, is an ER lipid scramblase

Lipid scramblases allow passive flip-flop of phospholipids between bilayer leaflets, thereby promoting membrane symmetry. At the endoplasmic reticulum (ER), where phospholipid synthesis is restricted to one of the two leaflets, scramblase activity should be essential for equilibrated membrane growth. However, phospholipid scramblases at the ER are poorly understood. The yeast protein Ist2 contains an ER domain and a cytosolic tail that binds the plasma membrane (PM) and participates in the transfer of phosphatidylserine (PS). Here, we show both in vitro and in silico that the ER- domain of Ist2, which bears homology to the TMEM16 proteins, possesses a lipid scramblase activity. Ist2 activity is not regulated by Ca2+, in contrast to TMEM16 proteins, but is affected by the lipid composition of the bilayer used in simulations. In cells, we do not find a strong impact of the scramblase domain of Ist2 in on PS distribution; however, its over-expression or deletion affects processes at the ER such as vesicular transport, lipid droplet biogenesis and general phospholipid transport, with a specific contribution of residues important for lipid scrambling. Our study therefore identifies the first dedicated phospholipid scramblase in yeast and demonstrates that membrane asymmetry can impact diverse membrane-remodeling processes at the ER.

cell biology↗

bioRxiv2021

Autoinhibition and regulation by phosphoinositides of ATP8B1, a human lipid flippase associated with intrahepatic cholestatic disorders

P4-ATPases flip lipids from the exoplasmic to the cytosolic leaflet, thus maintaining lipid asymmetry in eukaryotic cell membranes. Mutations in several human P4-ATPase genes are associated with severe diseases, e.g. in ATP8B1 causing progressive familial intrahepatic cholestasis, a rare inherited disorder progressing toward liver failure. ATP8B1 forms a binary complex with CDC50A and displays a broad specificity to glycerophospholipids, but regulatory mechanisms are unknown. Here, we report functional studies and the cryo-EM structure of the human lipid flippase ATP8B1-CDC50A at 3.1 [A] resolution. We find that ATP8B1 is autoinhibited by its N- and C-terminal tails, which form extensive interactions with the catalytic sites and flexible domain interfaces. Consistently, ATP hydrolysis is unleashed by truncation of the C-terminus, but also requires phosphoinositides, most markedly phosphatidylinositol-3,4,5-phosphate (PI(3,4,5)P3), and removal of both N- and C- termini results in full activation. Restored inhibition of ATP8B1 truncation constructs with a synthetic peptide mimicking the C-terminal segment further suggests molecular communication between N- and C-termini in the autoinhibition and demonstrates that the regulatory mechanism can be interfered with by exogenous compounds. A recurring (G/A)(Y/F)AFS motif of the C-terminal segment suggests that this mechanism is employed widely across P4-ATPase lipid flippases in plasma membrane and endomembranes.

biochemistry↗

bioRxiv2020

The SERCA residue Glu340 mediates inter-domain communication that guides Ca2+ transport

The sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) is a P-type ATPase that transports Ca2+ from the cytosol into the SR/ER lumen, driven by ATP. This primary transport activity depends on tight coupling between movements of the transmembrane helices forming the two Ca2+ binding sites and of the cytosolic headpiece mediating ATP hydrolysis. We have addressed the molecular basis for this intramolecular communication by analyzing the structure and functional properties of the SERCA mutant E340A. The mutated Glu340 residue is strictly conserved among the P-type ATPase family of membrane transporters and is located at a seemingly strategic position at the interface between the phosphorylation domain and the cytosolic ends of five out of SERCAs ten transmembrane helices. The mutant displays a marked slowing of the Ca2+-binding kinetics, and its crystal structure in the presence of Ca2+ and ATP analogue reveals a rotated headpiece, altered connectivity between the cytosolic domains and altered hydrogen bonding pattern around residue 340. Supported by molecular dynamics simulations, we conclude that the E340A mutation causes a stabilization of the Ca2+ sites in a more occluded state, hence displaying slowed dynamics. This finding underpins a crucial role of Glu340 in inter-domain communication between the headpiece and the Ca2+-binding transmembrane region.

molecular biology↗