Kon-tiki enhances PS2 integrin adhesion and localizes its ligand, Thrombospondin, in the myotendinous junction

Cell-extracellular-matrix adhesion is mediated by cell receptors, mainly integrins and transmembrane proteoglycans, which can functionally interact. How these receptors are regulated and coordinated is largely unknown. We show that the conserved transmembrane Drosophila proteoglycan Kon-tiki (Kon, also known as Perdido) interacts with the alpha PS2 beta PS integrin ( alpha PS2 is encoded by inflated and beta PS by myospheroid) to mediate muscle-tendon adhesion. kon and inflated double mutant embryos show a synergistic increase in muscle detachment. Furthermore, Kon modulates alpha PS2 beta PS signaling at the muscle attachment, since phosphorylated Fak is reduced in kon mutants. This reduction in integrin signaling can be rescued by the expression of a truncated Kon protein containing its transmembrane and extracellular domains, suggesting that these domains are sufficient to mediate this signaling. We show that these domains are sufficient to properly localize the alpha PS2 beta PS ligand, Thrombospondin, to the muscle attachment, and to partially rescue Kon-dependent muscle-tendon adhesion. We propose that Kon can engage in a protein complex with alpha PS2 beta PS and enhance integrin-mediated signaling and adhesion by recruiting its ligand, which would increase integrin- binding affinity to the extracellular matrix, resulting in the consolidation of the myotendinous junction.