Proper control of the mitochondrial Ca uniporter's pore (MCU) is required to allow Ca-dependent activation of oxidative metabolism and to avoid mitochondrial Ca overload and cell death. The MCU's gatekeeping and cooperative activation is mediated by the Ca-sensing MICU1 protein， which has been proposed to form dimeric complexes anchored to the EMRE scaffold of MCU. We unexpectedly find that MICU1 suppresses inhibition of MCU by ruthenium red/Ru360， which bind to MCU's DIME motif， the selectivity filter. This led us to recognize in MICU1's sequence a putative DIME interacting domain (DID)， which is required for both gatekeeping and cooperative activation of MCU and for cell survival. Thus， we propose that MICU1 has to interact with the D-ring formed by the DIME domains in MCU to control the uniporter.