A dual topology of STIM1 at the plasma membrane regulates calcium constitutive entry

STIM1, a type I transmembrane protein, was initially identified as a plasma membrane- localized protein with tumor growth suppressor activity, characterized by its N-terminal domain being extracellular (STIM1PM). The presence of STIM1PM at the plasma membrane (PM) is associated with N-glycosylation, which facilitates its trafficking and stability at the PM. Subsequently, STIM1PM has been recognized as a critical component in the regulation of store-independent Ca2+ entry pathways, including arachidonic acid-regulated Ca2+ (ARC) channels and constitutive Ca2+ entry (CCE). In this study, we demonstrate that STIM1PM exhibits dual topology at the PM, presenting both the expected type I orientation and an alternative type II orientation. We found that both orientations of STIM1PM contribute to the regulation of CCE. As anticipated, the N-glycosylation state of STIM1PM influences its presence at the PM and its dual topology. Our findings reveal that STIM1PM exists with dual topology and begin to elucidate the mechanisms underlying this multiple orientation.