Exposure of phosphatidylserine on the cell surface

Abstract

Phosphatidylserine (PtdSer) is a phospholipid that is abundant in eukaryotic plasma membranes. An ATP-dependent enzyme called flippase normally keeps PtdSer inside the cell, but PtdSer is exposed by the action of scramblase on the cell's surface in biological processes such as apoptosis and platelet activation. Once exposed to the cell surface, PtdSer acts as an 'eat me' signal on dead cells, and creates a scaffold for blood-clotting factors on activated platelets. The molecular identities of the flippase and scramblase that work at plasma membranes have long eluded researchers. Indeed, their identity as well as the mechanism of the PtdSer exposure to the cell surface has only recently been revealed. Here, we describe how PtdSer is exposed in apoptotic cells and in activated platelets, and discuss PtdSer exposure in other biological processes.

Figure 1

Structure of flippases and its cleavage by caspase. The structure of ATP11A/ATP11C and CDC50A is schematically shown. ATP11A and ATP11C carry 10 transmembrane segments. The ATPase domain in the cytoplasm is divided into A, actuator; N, nucleotide-binding; P, phosphorylation domains. CDC50A carrying two transmembrane regions functions as a chaperone for proper localization of ATP11A and ATP11C at plasma membranes. CDC50A forms a complex with ATP11A or ATP11C in the plasma membrane, and may be necessary for the flippase activity. ATP11A and ATP11C contain 2 and 3 caspase-recognition sites (Red) that flank a α-helix (Blue), and are cleaved during apoptosis for the PtdSer exposure. TAT-1 and TAT-3, P4-ATPases in C. elegans also carry putative caspase-recognition sites in the corresponding positions. DTKT, the conserved phosphorylation site, is underlined

Figure 2

Structure of TMEM16F. (a) A schematic representation of TMEM16F. The scrambling domain (SCRD) between transmembrane regions IV and V, and the Ca²⁺-binding site comprised of Aspartate and Glutamate in transmembrane regions VI–VIII are shown by red asterisks. (b) A tertiary structure of TMEM16 from N. haematococca. From the study by Brunner et al.

Figure 3

The molecular mechanism for PtdSer exposure in cells with high Ca²⁺-concentration. The flippase comprised of P4-ATPase (ATP11A or ATP11C) and CDC50A, and a Ca²⁺-dependent scramblase (TMEM16F) are schematically shown. In activated platelets, the intracellular Ca²⁺ concentration increases and activates TMEM16F to scramble phospholipids, while it inactivates P4-ATPases and reduces their flipping activity. When the Ca²⁺ concentration returns to normal level, TMEM16F stops scrambling phospholipids, while P4-ATPases resume flipping PtdSer and PtdEtn. Thus, PtdSer is only transiently exposed to the cell surface in this process, and likely depends on the intracellular concentration of ATP and Ca²⁺. The constant flipping of PtdSer prevents the PtdSer-exposing cells to be engulfed by macrophages

Figure 4

Structure of Xkr8, and its caspase-cleavage site. (a) The structure of mouse Xkr8, a caspase-dependent phospholipid scramblase is schematically shown. The caspase-recognition site at C-terminal region is indicated. In addition to Xkr8, Xkr4 and Xkr9 also carry the caspase-recognition site at the C-terminal tail region, and work as a caspase-dependent scramblase. (b) The amino-acid sequences of mouse Xkr8 (mXkr8), CED8 of C. elegans and CG32579 of Drosophila are aligned to obtain the maximal homology. The caspase-recognition sites at the C-terminal region of mXkr8 and at the N-terminal region of CED8 are highlighted in green. Transmembrane regions are underlined. reproduced from Suzuki et al. with permission

Figure 5

The PtdSer exposure in apoptotic cells. A caspase-dependent phospholipid scramblase of Xkr8 and flippase (ATP11A/ATP11C associated with CDC50A) are schematically shown. When cells undergo apoptosis, caspase 3 or caspase 7 in the downstream of the caspase cascade cleaves Xkr8 to activate its scramblase activity, while the same caspases cleave and inactivate ATP11A and ATP11C. This is the irreversible process, and the PtdSer exposed on the cell surface is recognized by macrophages for engulfment