Abstract Body (Do not enter title and authors here): Apolipoprotein A1 (APOA1), the most abundant protein in high-density lipoproteins (HDL), is the primary activator of lecithin cholesterol acyl transferase (LCAT). LCAT’s cholesteryl esterification activity is key for maturing nascent HDL discs into the spherical forms commonly found in plasma. However, the molecular mechanism for APOA1 activation of LCAT is poorly understood. Using phage display selection against nanodiscs (lipid complexes stabilized by an APOA1 variant commonly used in transmembrane protein structural biology), we have identified, isolated, and expressed a novel Fab fragment (Fab16). Size exclusion chromatography elution shift experiments showed that Fab16 readily bound reconstituted HDL(rHDL) particles containing human purified plasma APOA1 with nearly quantitative binding at a 1:1 molar ratio of Fab16 to APOA1. Negative stain electron microscopy (NSEM), confirmed that 1-2 Fab16 bound to each rHDL particle in a specific pattern around the disc periphery. Additionally, Fab16 effectively bound both large and small spherical HDL particles isolated from human plasma. Chemical cross-linking / mass spectrometry experiments indicated that Fab16 binds an APOA1 epitope between residues 94 and 118 (i.e., within amphipathic helix 4). As our previous work has shown that APOA4 helix 4 is important for LCAT activation, we hypothesized that Fab16 would inhibit LCAT activity. Using a novel gas chromatography cholesterol esterification assay, we found that Fab16 applied at a 1.5:1 molar ratio to APOA1 inhibited LCAT cholesterol esterification activity by 56±7%. The symmetrical nature of both discoidal and spherical HDL particles has hampered the application of cryogenic electron microscopy (cryo EM) for understanding HDL structure. Fab16 may provide a valuable ‘landmark’ to assist in deriving a high-resolution structure of the complex. Work to optimize conditions for collecting high quality cryo EM datasets of discoidal and spherical HDL in the presence and absence of LCAT is ongoing. Deriving a high-resolution understanding of HDL structure and its interaction with remodeling factors such LCAT has been a long-standing problem but is crucial for deriving new strategies to manipulate HDL function for therapeutic benefit.