This proposal aims to investigate the assembly mechanisms, structures, and functions of polymeric (p) immunoglobulins (Ig) that populate the mucosa. The pIgs are found in vertebrates and together form a structurally diverse group of antibodies. They comprise several Ig heavy chain classes, including mammalian IgA and IgM, which typically contain between two and five Ig monomers and one joining chain (JC); however, potential to assemble with the JC and/or to assemble into polymers of different size varies with vertebrate species and Ig heavy chain class. Following assembly, pIgs are transported to the mucosa by the polymeric Ig receptor (pIgR). In the mucosa, the pIgR ectodomain, called secretory component (SC), remains bound and the complex is referred to as a secretory (S) Ig. SIgA is the predominant mucosal antibody in mammals; it is typically found in dimeric (d) forms; however higher order polymers such as tetramers are functionally relevant. SIgA is associated with unique effector functions compared to monomeric, circulatory antibodies; it can coat, cross-link and agglutinate commensal and pathogenic antigens and also mediate interactions with receptors on host and microbial cells. Despite significance, the structural basis for pIg assembly and SIg functions remained poorly understood through decades of immunological research. In 2020 the cryo-electron microscopy structures of SIgM, SIgA and a dimeric (d) IgA precursor were published revealing unprecedented molecular insights into these crucial complexes and opening the door to new questions and structure-guided experiments. The structures of dIgA and dimeric forms of SIgA revealed two IgAs joined through the JC to form a pseudosymmetric, bent conformation that appears to restrict the positions of antigen-binding fragments (Fabs) and promote access to receptor-binding sites. The SC is asymmetrically bound to one side and is solvent accessible, suggesting it may promote yet uncharacterized interactions with host or microbial factors. These observations raise the questions of how structural differences among pIg are generated (e.g dimer versus tetramer and JC versus no JC), how the bent, asymmetric arrangement of components is induced and maintained, and how it contributes to function. The proposed research program will use structural and biophysical approaches to target these questions. Aim 1 will identify Ig heavy chain residues, structural motifs and/or conformational changes that promote pIg assembly and control pIg polymeric state, while also determining the structural basis for JC- independent pIg assembly and function. Aim 2 will characterize JC-specific mechanisms of pIg assembly and its structural contributions to the pseudosymmetric conformation of dIgA. Aim 3 will characterize the functional significance SC and its capacity to bind microbial ligands. These studies will deliver comprehensive mechanistic models for pIg assembly, generate new pIg structures and report new SIg structure...