Essential component of the TIM23 complex, a complex that mediates the translocation of transit peptide-containing proteins across the mitochondrial inner membrane. Component of the TIM23 complex, at least composed of TIM23, TIM17, TIM50 and TIM21. The complex interacts with the TIM44 component of the PAM complex. Around 20 amino acids of the N-terminus are believed to span the mitochondrial outer membrane. This association is dynamic and depends on the translocation activity of the TIM23 complex. However, this topology seems not to be critical for formation of a TOM-TIM supercomplex and preprotein import. Present with 1440 molecules/cell in log phase SD medium. Belongs to the Tim17/Tim22/Tim23 family.