Julie Eichler

La protéine STARD3 est sur-exprimée dans les cancers du sein HER2+ et participe à leur croissance. Cette protéine modulaire comporte un domaine MENTAL d’ancrage aux endosomes tardifs/lysosomes (ET/Lys), un motif Phospho-FFAT et un domaine START de transport de cholestérol. La phosphorylation du motif Phospho-FFAT de STARD3 active son interaction avec le domaine MSP des protéines VAP (VAPA, VAPB et MOSPD2) localisées au Réticulum Endoplasmique (RE), permettant la formation de Sites de Contacts Membranaires (SCM) RE- ET/Lys. La kinase GSK3 est responsable de cette phosphorylation et régule l’interaction de STARD3 avec les protéines VAP et les SCM entre le RE et les ET/Lys. En absence de phosphorylation, les ET/Lys s’agrègent en région périnucléaire. Ce phénotype dépend aussi du domaine START, dont nous avons identifié une nouvelle fonction : il est capable d’interagir avec les membranes chargées négativement, et est responsable du regroupement des ET/Lys positifs pour STARD3 lorsque cette dernière n’est pas engagée dans un contact RE-ET/Lys. Ainsi, la phosphorylation de STARD3 par GSK3 contrôle la formation de SCM et le positionnement des ET/Lys.

Membrane contact sites (MCSs) between the endoplasmic reticulum (ER) and other organelles or the plasma membrane play essential roles in lipid exchange and organelle homeostasis; however, how the molecular composition of these sites is functionally specified remains poorly understood. Here, we identify the ER-resident protein MOSPD2 as a novel regulator of late endosome/lysosome (LE/Lys) homeostasis and show that MOSPD2 selectively recruits the cholesterol transporter STARD3 at ER-LE/Lys contacts. Loss of either MOSPD2 or STARD3 leads to an expansion of the LE/Lys compartment, accumulation of free cholesterol at LE/Lys, and impaired LE/Lys fusion dynamics, indicating that both proteins act within a common pathway at ER-LE/Lys contact sites. STARD3 interacts with three members of the MSP-domain protein family, VAP-A, VAP-B and MOSPD2, yet, these tethers are not functionally equivalent at ER-LE/Lys MCSs. Using a series of quantitative binding assays, we show that STARD3 preferentially associates with MOSPD2 rather than with VAP-A or VAP-B, and that neither VAP-A nor VAP-B can compensate for the loss of MOSPD2 to restore LE/Lys biology and cholesterol distribution. This work establishes the MOSPD2-STARD3 complex as a unique functional unit that regulates LE/Lys homeostasis. Collectively, these findings reveal that selective pairing between MSP-domain proteins and their FFAT-containing partners exist and shape the interaction landscape of MCSs to define their functions.

Membrane contact sites between organelles are organized by protein bridges. Among the components of these contacts, the VAP family comprises ER–anchored proteins, such as MOSPD2, that function as major ER–organelle tethers. MOSPD2 distinguishes itself from the other members of the VAP family by the presence of a CRAL-TRIO domain. In this study, we show that MOSPD2 forms ER–lipid droplet (LD) contacts, thanks to its CRAL-TRIO domain. MOSPD2 ensures the attachment of the ER to LDs through a direct protein–membrane interaction. The attachment mechanism involves an amphipathic helix that has an affinity for lipid packing defects present at the surface of LDs. Remarkably, the absence of MOSPD2 markedly disturbs the assembly of lipid droplets. These data show that MOSPD2, in addition to being a general ER receptor for inter-organelle contacts, possesses an additional tethering activity and is specifically implicated in the biology of LDs via its CRAL-TRIO domain.

Membrane contact sites between organelles are organized by protein bridges. Among the components of these contacts, the VAP family comprises endoplasmic reticulum (ER)-anchored proteins, such as MOSPD2, functioning as major ER-organelle tethers. MOSPD2 distinguishes itself from the other members of the VAP family by the presence of a CRAL-TRIO domain. In this study, we show that MOSPD2 forms ER-LD contacts thanks to its CRAL-TRIO domain. MOSPD2 ensures the attachment of the ER to LDs through a direct protein-membrane interaction involving an amphipathic helix that has an affinity for lipid packing defects present at the surface of LDs. Remarkably, the absence of MOSPD2 markedly disturbs the assembly of lipid droplets. These data show that MOSPD2, in addition to being a general ER receptor for inter-organelle contacts, possesses an additional tethering activity and is specifically implicated in the biology of LDs via its CRAL-TRIO domain.