Many animals use the presence of the phospholipid phosphatidylserine (PS), on the outer leaflet of the plasma membrane, as a way to recognize and destroy apoptotic cells by phagocytic engulfment. In this issue, two papers illustrate the differential roles played by PS in normal cells and during virus infection (see the Perspective by Fairn and Grinstein). Darland-Ransom et al. identified an enzyme in Caenorhabditis elegans, aminophospholipid translocase 1 (TAT-1), which appears normally to restrict PS to the inner side of the plasma membrane. Animals lacking TAT-1 had increased PS on the cell surface, and random cells were lost from the animals in a process that depended on PSR-1, a PS receptor involved in the clearance of apoptotic cells. Mercer and Helenius used live-cell imaging to follow vaccinia virus entry into tissue culture cells. Viruses first bound to actin-rich cell-surface protrusions, filopodia, along which they surfed to the cell body. At the cell body, the incoming virus stimulated its own uptake due to the presence of PS on the viral membrane, mimicking the uptake of apoptotic cell corpses.
M. Darland-Ransom, X. Wang, C.-L. Sun, J. Mapes, K. Gengyo-Ando, S. Mitani, D. Xue, Role of C. elegans TAT-1 protein in maintaining plasma membrane phosphatidylserine asymmetry. Science 320, 528-531 (2008). [Abstract] [Full Text]