SARAF Luminal Domain Structure Reveals a Novel Domain-Swapped β-Sandwich Fold Important for SOCE Modulation

Abstract Store-Operated Calcium Entry (SOCE) plays key roles in cell proliferation, muscle contraction, immune responses, and memory formation. The coordinated interactions of a number of proteins from the plasma and endoplasmic reticulum membranes control SOCE to replenish internal Ca 2+ stores and generate intracellular Ca 2+ signals. SARAF, an endoplasmic reticulum resident component of the SOCE pathway having no homology to any characterized protein, serves as an important brake on SOCE. Here, we describe the X‐ray crystal structure of the SARAF luminal domain, SARAF L . This domain forms a novel 10-stranded β-sandwich fold that includes a set of three conserved disulfide bonds, denoted the “SARAF-fold.” The structure reveals a domain-swapped dimer in which the last two β-strands (β9 and β10) are exchanged forming a region denoted the “SARAF luminal switch” that is essential for dimerization. Sequence comparisons reveal that the SARAF-fold is highly conserved in vertebrates and in a variety of pathologic fungi. Forster resonance energy transfer experiments using full-length SARAF validate the formation of the domain-swapped dimer in cells and demonstrate that dimerization is reversible. A designed variant lacking the SARAF luminal switch shows that the domain swapping is essential to function and indicates that the SARAF dimer accelerates SOCE inactivation.