Abstract

Contact sites are specialized zones of proximity between organelles, essential for communication and coordination. The formation of Endoplasmic Reticulum (ER) contact sites, relies on a unique membrane environment enriched in sterols. However, how these sterol-rich domains are formed and maintained was unclear. We found that the yeast membrane protein Yet3, homolog of human BAP31 , localizes to multiple ER contacts. We show that Yet3 interacts with the post-squalene ergosterol biosynthesis enzymes through its transmembrane domains. Yet3 recruits the enzymes to create sterol-rich environments. Increasing sterols at ER contacts causes their depletion from the plasma membrane, leading to a compensatory reaction and altered cell metabolism. We demonstrate that the molecular function of Yet3 and BAP31 is conserved. Our data shows that Yet3 and BAP31 provide on-demand sterols at contacts thus shaping organellar structure and function. This molecular understanding provides new insights into the role of BAP31 in development and pathology.