The guidance and adhesion protein FLRT2 dimerizes in cis via dual Small-X3-Small transmembrane motifs

<jats:p>Fibronectin Leucine-rich Repeat Transmembrane (FLRT 1-3) proteins are a family of broadly expressed single-spanning transmembrane receptors that play key roles in development. Their extracellular domains mediate homotypic cell-cell adhesion and heterotypic protein interactions with other receptors to regulate synapse development and cell guidance. These in trans FLRT interactions determine the formation of signaling complexes of varying complexity and function. Whether FLRTs also interact in cis is remains unknown. Here, we reveal two dimerization motifs in the FLRT2 transmembrane helix. Molecular dynamics simulations and single particle tracking experiments show that these 'Small-X3-Small' motifs synergize with a third dimerization motif encoded in the extracellular domain to permit the cis association and diffusion patterns of FLRT2 on cells. The results point to a competitive switching mechanism between in cis and in trans interactions which suggests that homotypic FLRT interaction mirrors the functionalities of classic adhesion molecules.</jats:p>