The biogenesis of exosomes, small secreted vesicles involved in signalling processes, remains incompletely understood. Here, we report evidence that the syndecan heparan sulphate proteoglycans and their cytoplasmic adaptor syntenin control the formation of exosomes. Syntenin interacts directly with ALIX through LYPX(n)L motifs, similarly to retroviral proteins, and supports the intraluminal budding of endosomal membranes. Syntenin exosomes depend on the availability of heparan sulphate, syndecans, ALIX and ESCRTs, and impact on the trafficking and confinement of FGF signals. This study identifies a key role for syndecan-syntenin-ALIX in membrane transport and signalling processes.

A talin intermolecular interaction autoinhibits its own activation and regulates β3-integrin binding. When bound, β3-integrin undergoes structural alterations that prevent its β and α subunits from associating, maintaining β3-integrin's clustering capability.

Syndecans are heparan sulfate proteoglycans that modulate the activity of several growth factors and cell adhesion molecules. PDZ domains in the adaptor protein syntenin interact with syndecans and with the phosphoinositide PIP(2), which is involved in the regulation of the actin cytoskeleton and membrane trafficking. Here, we show that the syntenin PDZ domain-PIP(2) interaction controls Arf6-mediated syndecan recycling through endosomal compartments. FGF receptor accompanies syndecan along the syntenin-mediated recycling pathway, in a heparan sulfate- and FGF-dependent manner. Syndecans that cannot recycle via this pathway become trapped intracellularly and inhibit cell spreading. This syntenin-mediated syndecan recycling pathway may regulate the surface availability of a number of cell adhesion and signaling molecules.

PDZ (Postsynaptic density protein, Disc large, Zona occludens) domains are protein-protein interaction modules that predominate in submembranous scaffolding proteins. Recently, we showed that the PDZ domains of syntenin-1 also interact with phosphatidylinositol 4,5-bisphosphate (PIP 2 ) and that this interaction controls the recruitment of the protein to the plasma membrane. Here we evaluate the general importance of PIP 2 -PDZ domain interactions. We report that most PDZ proteins bind weakly to PIP 2 , but that syntenin-2, the closest homolog of syntenin-1, binds with high affinity to PIP 2 via its PDZ domains. Surprisingly, these domains target syntenin-2 to nuclear PIP 2 pools, in nuclear speckles and nucleoli. Targeting to these sites is abolished by treatments known to affect these PIP 2 pools. Mutational and domain-swapping experiments indicate that high-affinity binding to PIP 2 requires both PDZ domains of syntenin-2, but that its first PDZ domain contains the nuclear PIP 2 targeting determinants. Depletion of syntenin-2 disrupts the nuclear speckles-PIP 2 pattern and affects cell survival and cell division. These findings show that PIP 2 -PDZ domain interactions can directly contribute to subnuclear assembly processes.