STIM1 is recognized as an ER Ca²⁺ sensor of calcium release-activated calcium (CRAC) channel that is constructed by membrane protein Orai1. However, this regulatory system may also be regulated by other proteins. Reticulocalbin 2 (RCN2) was purified and identified from STIM1-Orai1 complex. Confocal microscopy revealed that RCN2 co-localized with STIM1 in ER before and after Ca²⁺ store depletion. Single cell [Ca²⁺]_i measurements of RCN2 EF hands mutant showed slight influence on SOC electrophysiological characters. Furthermore, a novel collar form aggregation of RCN2 surrounding STIM1 clusters suggested that RCN2 potentially plays a role of structure maintenance in STIM1 clustering.