Figure 2.

The function of PS. (A) PS acts as a signal to be engulfed by macrophages with two recognition modes. One is directly recognized by phagocytic receptors BAI1, TIM-4, and Stabilin 2. The other is indirect recognition. The bridging molecules MFG-E8 (also known as lactadherin) and GAS6 bind to PS, which is recognized by membrane proteins MER and α_Vβ₃. These two recognition patterns are not mutually exclusive and may co-occur. (B) PS provides a negatively charged surface to initiate and maintain coagulation. In normal conditions, PS is sequestered in the inner layer of cell membranes by the action of floppase and flippase. When intracellular Ca²⁺ increases, the ATP-dependent translocation enzyme is blocked, and the scramblase is activated, resulting in a random distribution of PS to both sides of the membrane. PS, initially located in the cell’s inner membrane, is exposed to the outer side. On the outer membrane, PS provides active clotting catalytic surfaces for forming TF-FVIIa complex, factor X-enzyme complex (FIXa-FVIIIa-Ca²⁺-PL), and prothrombinase complex (FXa-FVa-Ca²⁺-PL).