Fld1/Ldb16 complex stabilizes and establishes a diffusion barrier at the ER–LD contact sites. (A) The scheme illustrates a contact site between the ER and a LD. Although connected with the ER membrane, the LD monolayer has different properties such as a higher surface tension (in orange). The presence of the Fld1/Ldb16 complex at the contact sites prevents the equilibration of the two membrane systems. (B) In the absence of this complex, phospholipids freely diffuse between the two organelles. Under low-synthesis conditions, phospholipids can become limiting and LDs coalesce into a supersized one. The mild surface tension (faint orange) in these LDs still allows the targeting of LD proteins, albeit at lower efficiency (left). Under high-phospholipid-synthesis conditions, ER membrane and LDs equilibrate. The low surface tension of these LDs prevents their coalescence and the recruitment of LD proteins. Instead, these aggregates display phospholipid packing defects (green), which recruit ALPS-motif–containing proteins (right). Both low and high phospholipid synthesis leads to phospholipid defects in membranes adjacent to LDs, which are recognized by proteins containing canonical AHs (brown).