Biological membranes are composed of a fluid lipid bilayer with proteins embedded therein. While third essential component, carbohydrates, attach with either lipids or proteins. The highly dynamic biological membranes contribute to the identity and integrity of the enclosed structures. The composition of biological membranes, particularly lipids, is critical to the cells' physical properties. Using Caenorhabditis elegans as a model system, we reviewed the significant roles of membrane lipids in regulating membrane homeostasis [1].
The glycerophospholipids (GPLs) are asymmetrically distributed across the bilayer. This lipid asymmetry is due to multiple factors, including two types of ATP-driven pumps: flippases and floppases. A third class of lipid transporter phospholipid scramblase mediates trans-bilayer phospholipids translocation leading to a rapid collapse of the membrane asymmetry. Scramblase was first identified from the reconstituted human erythrocyte membranes. Later, four homologs of human scramblases, hPLSCR1 – 4 with different subcellular localization and function, were identified. The scramblase orthologs are also identified in mice, rats, Drosophila, Caenorhabditis elegans, and Saccharomyces cerevisiae, indicating that the gene is well conserved through evolution. A BLAST search identified eight PL scramblases, SCRM-1 through SCRM-8 in Caenorhabditis elegans. Sequence alignment of these eight scramblases with hPLSCR1 revealed that SCRM-1 shares 41% sequence identity and 57% sequence similarity with hPLSCR1. With spectroscopic studies, we identified SCRM-1 of Caenorhabditis elegans as a phospholipid scramblase facilitating head group independent trans-bilayer phospholipid translocation [2].
Though scramblase performs essential functions, very little is known about their regulation. With in-silico analysis, we identified a CRAC (Cholesterol Recognition Amino Acid Consensus) motif at the C-terminal of SCRM-1. The SCRM-1 exhibited strong interaction with cholesterol via the putative cholesterol binding CRAC motif. Such interaction in the artificial membrane attenuated the scramblase activity of SCRM-1 [3]. Finally, the in vivo effects of cholesterol-scramblase interaction were investigated in mammalian cell lines using human phospholipid scramblase 1 (hPLSCR1). Inhibition of such interaction by mutations in the putative CRAC motif or depletion of plasma membrane cholesterol resulted in the nuclear import of hPLSCR1 (human PL scramblase 1). Inside the nucleus, hPLSCR1 induces IP3R1 expression leading to the excessive calcium release from ER. Moreover, the expression of hPLSCR1, especially its CRAC mutants, induced apoptosis. The study suggested a plausible link between depletion of plasma membrane cholesterol, the expression of hPLSCR1, intracellular Ca2+ release, and apoptosis (Revised manuscript submitted to International Journal of Biological Macromolecules-Under review).
Publications:
[1] Koyiloth M, Gummadi S N, Regulation and Function of Membrane Lipids: Insights from Caenorhabditis elegans. BBA Advances. 2, 2022, 100043 (Review article), 01/2022.

[2] Koyiloth M, Gummadi S N, Molecular cloning and biochemical characterization of the phospholipid scramblase SCRM-1 from Caenorhabditis elegans. Eur Biophys J. 2020 Mar;49(2):163-173, 2020

[3] Koyiloth M, Gummadi S N, Cholesterol interaction attenuates scramblase activity of SCRM-1 in the artificial membrane. Biochimica et Biophysica Acta (BBA) - Biomembranes. 1863(9):183548, 2021

Under Review
[1] Koyiloth M, Gummadi S N, Human Phospholipid Scramblase 1-Cholesterol Interaction via CRAC Motif is Essential for Functional Regulation and Subcellular Localization. International Journal of Biological Macromolecules. 2022