Effect of an Apoptotic Membrane raft on the Conformational and Dynamical Changes of Calreticulin

Abstract

Calreticulin (CRT) on the cell surface can mediate engulfment of apoptotic cells by interacting directly with LDL receptor-related protein (LRP1) (Cell. 123:321-34, 2005). Phosphatidyl serine (PS) lipids in the inner leaflet of the cell membrane are externalized and become exposed in cholesterol-rich domains during apoptosis and co-localized with cell surface surface CRT (Cell. 123:321-34, 2005). How the apoptotic cell membrane affects the structure and dynamics of CRT, further influencing CRT binding to LRP1 to signal apoptotic-cell clearance remain unknown. In this study, we investigated the interactions of a membrane raft in an apoptotic cell membrane with CRT and its effect on conformational and dynamical changes in CRT via atomically detailed molecular dynamics simulations. An apoptotic membrane raft membrane is modeled as a bilayer containing 1-palmitoyl-2-oleoyl-sn-glycero-3phosphocholine (POPC) lipids, cholesterol (CHOL) and palmitoyl-oleoyl phosphatidylserine (POPS) lipids (the ratio of the number of POPC lipid, POPS lipid and CHOL is: 5:1:4). The results are compared with the membrane raft without POPS lipids. POPS lipids in the membrane raft affected the microscopic and mesoscopic properties of the membrane raft. Apoptotic membrane raft resulted in the more direct interaction of the N-, partial C, and P-domains of CRT with membrane raft and more stabilized CRT conformation compared to that of CRT in a POPC-CHOL membrane raft, which could affect CRT recruitment of LRP1. Results from this study provided molecular insight into the effect of apoptotic membrane raft on CRT conformational and dynamical changes, further affecting CRT binding to LRP1 for signaling.Keywords: membrane raft, POPS lipids, TSP1-CRT complex, apoptotic cell clearance, molecular dynamics simulations