Structural and Biochemical Studies of the Human Mitochondrial Calcium Uniporter Complex

Abstract

Cellular Ca2+ homeostasis is tightly regulated by the mitochondrial calcium uniport (MCU) complex, which transports Ca2+ from the intermembrane space (IMS) to the matrix of mitochondria. The mitochondrial Ca2+ influx via the MCU is mediated by mitochondrial calcium uptake protein 1 and 2 (MICU1 and MICU2) found on the IMS. In this study, I determined the crystal structure of the human MICU1-MICU2 heterodimer in an apo-state which serves as the MCU gatekeeper. The MICU1 and MICU2 assemble in the face-to-face heterodimer with salt bridges and methionine knobs stabilizing the heterodimer in an apo state. Structural analysis suggested how the MICU1-MICU2 heterodimer sets a higher threshold for Ca2+ uptake than MICU1 homodimer. In addition to the MICU proteins, several studies have revealed that phosphorylation of the MCU could also affect the MCU activity. To elucidate the kinase responsible for phosphorylating a highly conserved phosphorylation target site (Ser92) present in the N-terminal domain of MCU, radioisotope labeling analysis was conducted, which identified protein kinase C as the primary kinase for phosphorylation of Ser92 of MCU. Structural and molecular dynamics simulation studies suggested that the Ser92 phosphorylation might reduce the MCU activity.