Regulation of mitochondrial protein transport: Tim23 as a link to the calcium-signalling network in the cell
Final Report Abstract
Many metabolic processes essential for plant viability take place in mitochondria. To ensure that the function of this organelle is carefully balanced in accordance with specific requirements of the cell, mitochondria are tightly integrated into diverse regulatory networks. We recently described an influence of calcium/calmodulin on protein import process into plant mitochondria at the level of TIM23 and TIM22 mediated translocation on the inner membrane. Here we showed that Tim23, the central pore of TIM23 translocase, might be the mediator of the calmodulin (CaM) regulation in Arabidopsis since we could identify ATTim23.2 as a CaM-binding protein. Furthermore, we showed that ATTim23.2 possesses two CaM-binding domains one at the N-terminus and the second within the first loop. Topology studies indicated that the first (N-terminal) CaM-binding domain is exposed in the intermembrane space and the second one is located within the matrix of mitochondria. Therefore, the results indicate that CaM regulation of ATTim23.2 could take place on both sides of the inner membrane of mitochondria: in the intermembrane space and in the matrix. ATCML30, a recently identified mitochondrial CaM-like protein, could present a potential interaction partner for ATTim23.2. We showed that ATCML30 is localized in the intermembrane space (and not the matrix) of mitochondria, indicating that ATCML30 might interact with ATTim23.2 at its N-terminally localized CaM-binding domain. The TIM23 translocon exists in two dynamic forms: "TIM23-SORT" and "TIM23-MOTOR". While TIM23-SORT ensures the insertion of imported proteins into the inner membrane of mitochondria, the TIM23-MOTOR mediates the transport of proteins across the inner membrane into the matrix. Thus, it is possible that the intermembrane space CaM-binding domain of ATTim23.2 might regulate "TIM23-SORT" while its matrix CaM-binding domain regulates "TIM23-MOTOR". A differential role of the two CaM-binding domains in the recruitment and in the transport of precursor proteins into and across the inner membrane of mitochondria could therefore be suggested. Further studies provided evidences for the dimerization of ATTim23.2. Such dimerization has been shown for Tim23 in yeast and seems to be a regulator for the opening and closing of the Tim23 pore. However, the function of dimerization of Tim23 in plants and the impact of CaM on this dimerization remains unclear.
Publications
- (2011) Calmodulin and Calmodulin-like proteins in Arabidopsis thaliana. Endocytobiosis and Cell Research 21, 114-121
Fuchs M., Vothknecht U.C. and Chigri F.
- (2011) The Arabidopsis calmodulin-like proteins AtCML30 and AtCML3 are targeted to mitochondria and peroxisomes, respectively. Plant Molecular Biology 78, 211–222
Chigri F., Flosdorff S., Pilz S., Kölle E., Dolze E., Gietl C. and Vothknecht U.C.
