Tim23, an essential component of the protein import machinery of the inner membrane of mitochondria (TIM complex), forms dimers that display a dynamic behavior. Dimer formation is promoted by the membrane potential Δω. Binding of a matrix targeting sequence to Tim23 triggers dimer dissociation. Monomeric Tim23 is present when a preprotein chain is in transit across the TIM complex. Dimerization of Tim23 is dependent on the second half of its N-terminal hydrophilic domain, which is exposed to the intermembrane space. This segment contains a heptad leucine repeat motif with a predicted capacity for dimer formation. We propose that Tim23 exerts a key function in protein import: Tim23 dimers formed in response to Δω act as receptors for matrix targeting sequences on the surface of the inner membrane. The ensuing dissociation of Tim23 dimer triggers opening of the TIM channel and insertion of the preprotein.
