Preliminary Structural and Biochemical Studies of the Human Calcium Binding Protein and Mitochondrial Calcium Uniporter Complex

Abstract

Calcium ion acts as a second messenger for various cellular processes and it is important to sense the calcium signal for proper cellular responses. These calcium signals are detected by intracellular calcium binding proteins (CBP). There are diverse proteins regulated by the CBP and it is involved in numerous cellular functions. However, the studies about mitochondrial CBP binding protein still limited. Recently, it has been reported that CBP affects the mitochondrial calcium uniporter (MCU), which is located in the mitochondrial inner membrane and transport calcium into the matrix, directly or indirectly. Based on these studies, we hypothesized that CBP could interact with MCU and regulate it in the matrix. In this study, the preliminary structural and biochemical studies have been performed to investigate the human CBP and MCU complex. The interaction between CBP and MCU was first discovered using biochemical studies. MCU and CBP interact through the coiled-coil 2 domain of MCU, which is consistent with the prediction, and they interact only in the presence of calcium. After mapping the CBP binding site of MCU, the crystals were obtained from the co-crystallization of CBP and MCU peptide and it was diffracted at 1.9 Å. However, the solved structure was peptide-free CBP. In addition, the modeling of CBP-MCU complex was helpful to provide insight into the molecular interaction and mechanism between CBP and MCU. This preliminary study suggests that CBP might act as a new regulator of MCU in the matrix and provides a framework to study the structural and functional roles of CBP-MCU complex.