The mitochondrial protein translocase (MPT) family, which brings nuclearly encoded preproteins into mitochondria, is very complex with 19 currently identified protein constituents. These proteins include several chaperone proteins, four proteins of the outer membrane translocase (Tom) import receptor, five proteins of the Tom channel complex, five proteins of the inner membrane translocase (Tim) and three "motor" proteins.The inner membrane translocase is formed of a complex with a number of proteins, including the Tim17, Tim23 and Tim44 subunits. Tim17 and Tim23 are thought to form the translocation channel of the inner membrane.