Profiling the LAM family of contact-site tethers provides mechanistic insights into their regulation and function

Membrane contact sites are molecular bridges between organelles that are sustained by tethering proteins and enable organelle communication. The endoplasmic reticulum (ER) membrane harbors many distinct families of tether proteins that enable the formation of contacts with all other organelles. One such example is the LAM (Lipid transfer protein At Membrane contact sites) family, composed of six members. All six LAM proteins share the presence of a sterol transfer domain and a transmembrane segment that anchors them to the ER. However, the six proteins are divided into three homologous pairs each unique in their molecular architecture and localization to different ER subdomains. What determines the distinct localization of the different LAMs and which specific roles they carry out in each contact are still open questions. To address these, we utilized a proximity labeling approach to profile the interaction landscape of the entire family. Focusing on unique interactors we could support a role for Lam5 at the ER-mitochondria contact site. Capturing shared interactors of multiple LAMs explains how Lam1/3 and Lam2/4 paralogous pairs could be associated specifically with the plasma membrane. Overall, our work provides new insights into the regulation and function of the pan-ER contact LAM family proteins. More globally it demonstrates how proximity labeling can help identify the shared or unique functions of paralogous proteins.