RESUMEN
Membrane contact sites (MCS) are intracellular regions where two organelles come closer to exchange information and material. The majority of the endoplasmic reticulum (ER) MCS are attributed to the ER-localized tether proteins VAPA, VAPB, and MOSPD2. These recruit other proteins to the ER by interacting with their FFAT motifs. Here, we describe MOSPD1 and MOSPD3 as ER-localized tethers interacting with FFAT motif-containing proteins. Using BioID, we identify proteins interacting with VAP and MOSPD proteins and find that MOSPD1 and MOSPD3 prefer unconventional FFAT-related FFNT (two phenylalanines [FF] in a neutral tract) motifs. Moreover, VAPA/VAPB/MOSPD2 and MOSPD1/MOSPD3 assemble into two separate ER-resident complexes to interact with FFAT and FFNT motifs, respectively. Because of their ability to interact with FFNT motifs, MOSPD1 and MOSPD3 could form MCS between the ER and other organelles. Collectively, these findings expand the VAP family of proteins and highlight two separate complexes in control of interactions between intracellular compartments.