Surf4 modulates STIM1-dependent calcium entry

► Surf4 was identified as a STIM1 binding partner. ► Surf4 interacts with STIM1 in the lumina of the ER. ► Surf4-deficeint DT40 B cells show a marked increase of SOCE. ► Absence of Surf4 facilitates STIM1 clustering after store-depletion. Store-operated Ca2+ entry (SOCE) is crucial for various physiological responses in immune cells. Although it is known that STIM1 relocates into discrete puncta juxtaposed to the plasma membrane to initiate SOCE, the machinery modulating the function of STIM1 remains unclear. We explored to find its modulators using affinity purification for STIM1-binding proteins and identified surfeit locus protein 4 (Surf4). Surf4 associated with STIM1 in the endoplasmic reticulum. Deletion of Surf4 in DT40 B cells resulted in marked increase of SOCE and facilitation of STIM1 clustering upon store-depletion. These findings suggest the modulatory function of Surf4 for STIM1-mediated SOCE.