Role of Tim21 in mitochondrial translocation contact sites

Translocation of preproteins with N-terminal presequences into mitochondria requires the cooperation of the translocase of the outer membrane (TOM complex) and the presequence translocase of the inner membrane (TIM23 complex). However, the molecular nature of the translocation contact sites is poorly understood. We have identified a novel component of the TIM23 translocase, Tim21, which is involved in their formation. Tim21 is anchored in the mitochondrial inner membrane by a single transmembrane domain and exposes its C-terminal domain into the intermembrane space. The purified C-terminal domain of Tim21 appears not to bind to any of the TIM23 components but rather specifically interacts with the TOM complex. We propose that Tim21 binds to the trans site of the TOM complex thus keeping the two translocases in close contact.