page_title: Phospholipid scramblase
section_title: Nuclear localisation sequence
hierarchical_section_title: Phospholipid scramblase / Nuclear localisation sequence
caption_attribution_description: English: Crystallographic structure of importin bound to the non-classical PLSCR1-NLS based on PDB: 1Y2A&#8203;
context_page_description: Scramblase is a protein responsible for the translocation of phospholipids between the two monolayers of a lipid bilayer of a cell membrane. In humans, phospholipid scramblases constitute a family of five homologous proteins that are named as hPLSCR1–hPLSCR5. Scramblases are not members of the general family of transmembrane lipid transporters known as flippases. Scramblases are distinct from flippases and floppases. Scramblases, flippases, and floppases are three different types of enzymatic groups of phospholipid transportation enzymes. The inner-leaflet, facing the inside of the cell, contains negatively charged amino-phospholipids and phosphatidylethanolamine. The outer-leaflet, facing the outside environment, contains phosphatidylcholine and sphingomyelin. Scramblase is an enzyme, present in the cell membrane, that can transport the negatively charged phospholipids from the inner-leaflet to the outer-leaflet, and vice versa.
context_section_description: Phospholipid scramblase 1 (PLSCR1), a lipid-binding protein that enters the nucleus via the nonclassical NLS (257)GKISKHWTGI(266). The structure of the nuclear localisation sequence of scramblase PLSCR1 complexed to importin was determined using X-ray diffraction with a resolution of 2.20 Ångströms.  It is found in most mammals including humans. The import sequence lacks a continuous stretch of positively charged residues, and it is enriched in hydrophobic residues. Thus, Scramblase can transport negatively charged phospholipids from the inside of the cell to the outside of the cell.  The importin structure is composed of many alpha helices that integrate the protein into membranes. The role of importin is to move proteins such as scramblase into the nucleus.
caption: Structure of mouse importin (rainbow colored cartoon, N-terminus = blue, C-terminus = red) bound the nuclear localisation sequence of PLSCR1 scramblase (magenta tube; left hand side of figure).[7]
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