Discussion

Small Tims (translocase of the mitochondrial inner membrane) make up a group of mitochondrial intermembrane space chaperones. Trypanosoma brucei, the protozoan parasite that causes African trypanosomiasis, possesses at least 3 small Tim proteins that include the homologs of Tim9 (TbTim9), Tim10 (TbTim10), and a third one that is homologous to both Tim8 and Tim13 (TbTim8/13). Each possesses a pair of CX3C motifs, a conserved feature required for oxidative folding of this protein family. We found that TbTim9 and TbTim10 are required for optimum cell growth and TbTim8/13 is essential for T. brucei survival. TbTim8/13 interacts with TbTim9 as well as TbTim10, however, TbTim9 and TbTim10 do not interact with each other, which may explain the essentiality of TbTim8/13. Knockdown of  the small TbTims reduces the steady state levels and assembly of mitochondrial carrier protein, MCP5, but do not have any effect on the import of the presequence-containing proteins. Interestingly, small TbTim knockdown increases the levels of reactive oxygen species up to 12 fold and reduces the stability of TbTim17, the major component of the TIM complex in T. brucei,  while import of TbTim17 is minimally affected. The stability of TbTim17 is similarly reduced when T. brucei is treated with paraquat. Together, our results show that small TbTims with differential interaction pattern than higher eukaryotes are critical for cellular redox homeostasis and TbTim17 acts as a redox-sensor in T. brucei.