Description
The bioactive lipid lysophosphatidic acid (LPA) is involved in a multitude of signaling pathways including those associated with inflammation, wound healing, arthritis, and cancer. A humanized monoclonal antibody raised against LPA has been generated which is capable of binding LPA with high specificity. The LT3015 antibody has many potential therapeutic applications. In order to gain detailed understanding of its mechanism we determined the structure of the apo Fab, Fab complexed with LPA(C14:0), and Fab complexed with LPA(C18:2) to 2.15 _, 1.98 _, and 2.50 _ respectively. LPA was found to be buried deep within the antibody antigen-binding site. Interactions are mediated predominantly by amino acids of heavy chain CDRs, which completely envelop the region of the lipid surrounding the phosphoglycerol head group, through the ester group, and even several of the acyl chain carbons. In contrast to the previously determined structure of the closely related lipid sphingosine-1-phosphate (S1P) and the antibody LT1009, the phosphoglycerol head of LPA was determined to be hydrogen bonded to both the heavy and light chains in a metal-independent fashion. In both the LPA and S1P complex structures a conserved heavy chain Tyr side chain contacts the peptide backbone creating a "latch" to help orient the antigenic acyl tails along a shallow hydrophobic channel. Overall these models show means by which antibodies can bind phospholipids in a metal-independent manner and the interactions that lead to specific binding of LPA.
