Molecular Insight for the Effect of Lipid Raft on Thrombospondin-1 and Calreticulin Interactions

Abstract

Thrombospondin-1 (TSP1) binding to calreticulin (CRT) on the cell surface stimulates association of CRT with LDL receptor-related protein (LRP1) to signal focal adhesion disassembly and engagement of cellular activities (J. Biol. Chem. 275:36358-68, 2000; J. Biol. Chem. 277: 37219-28, 2002). Study demonstrated that lipid rafts are necessary for TSP1-mediated focal adhesion disassembly (J. Biol. Chem. 279, 23510-16, 2004), but the molecular mechanism of the phenomenon is still unknown. In this study, we investigated the interactions of a lipid bilayer and a lipid raft with CRT and TSP1-CRT complex and their effects on the structural changes of CRT and TSP1-CRT complex via atomically detailed molecular dynamics simulations. The lipid bilayer was modeled as a 1-palmitoyl-2-oleoyl-sn-glycero-3phosphocholine (POPC) bilayer with 1152 lipids. The lipid raft was modeled as a bilayer of POPC lipids mixed with cholesterol (CHOL) (40% of CHOL molecules in the lipid raft) or with both CHOL and sphingomyelin (SM) (the ratio of the number of POPC lipid, CHOL and SM is: 3:4:3). Results showed that TSP1 binding to CRT resulted in a more “open” conformation for CRT P-domain with respect to the CRT N-domain compared to that of single CRT in a lipid raft environment, but not in a POPC bilayer environment. Sphingomyelin enhanced the “open” CRT conformation by TSP1, which could expose the potential binding site(s) in CRT for binding to LRP1 to signal focal adhesion disassembly. Results also showed that micro- and mesoscopic properties of a lipid raft were significantly different from a POPC bilayer, which could also affect cell surface CRT interactions with TSP1. Results from this study provided molecular insight for the effect of lipid raft on TSP-CRT interactions and CRT-mediated focal adhesion disassembly.