Translocase of outer mitochondrial membrane 40 (Tom40)
Tom40 forms a channel in the mitochondrial outer membrane with a pore about 1.5 to 2.5 nanometers wide. It functions as a transport channel for unfolded protein chains and forms a complex with Tom5, Tom6, Tom7, and Tom22. The primary receptors Tom20 and Tom70 recruit the unfolded precursor protein from the mitochondrial-import stimulating factor (MSF) or cytosolic Hsc70. The precursor passes through the Tom40 channel and through another channel in the inner membrane, formed by Tim23, to be finally translocated into the mitochondrial matrix. The process depends on a proton motive force across the inner membrane and requires a contact site where the outer and inner membranes come close. Tom40 is also involved in inserting outer membrane proteins into the membrane, most likely not via a lateral opening in the pore, but by transfering precursor proteins to an outer membrane sorting and assembly machinery.