Formins are putative actin polymerization proteins involved in several actin-dependent processes, including cell migration, stress fiber formation, and formation of the cytokinetic ring. Though many members of the formin family have been characterized in terms of their structure, function, and regulatory roles, the mammalian formin known as FHOD3 (Formin Homology Domain 2 Containing 3) has been one of the lesser-studied members, with much information about its biological roles still to be uncovered. Our lab has previously observed that knocking down FHOD3 in vitro disrupts endothelial cell adherens junctions (a form of cell-to- cell connection), resulting in a more porous cell monolayer. Given that endothelial cells form the single-cell lining of blood capillaries, this finding implicates FHOD3 in the control of cell permeability by way of adherens junction remodeling. FHOD3 in its resting state is autoinhibited by forming a head-to- tail loop structure, therefore our lab has created a constitutively active form of FHOD3 by removing part of the C-terminus, in order to further study its activities in the cell. We plan to employ a BioID screen, whereby the constitutively active version of FHOD3 will be fused with a promiscuous biotin ligase (BioID), allowing FHOD3-BioID to biotinylate any proteins with which it interacts following transfection and expression. All biotinylated proteins will then be immunoprecipitated and analyzed by mass spectrometry to obtain a list of FHOD3-interactors, providing insight into potential regulatory interactions of FHOD3 in the context of adherens junctions. This insight will improve our current understanding of regulation of endothelial cell permeability at the level of adherens junctions, and possible models of dysregulation in diseased states, such as in atherosclerosis.