TIM21 interacts with the outer mitochondrial TOM complex and promotes the insertion of proteins into the inner mitochondrial membrane [1].

1: Mitochondrial presequence translocase: switching between TOM tethering and motor recruitment involves Tim21 and Tim17. Chacinska A, Lind M, Frazier AE, Dudek J, Meisinger C, Geissler A, Sickmann A, Meyer HE, Truscott KN, Guiard B, Pfanner N, Rehling P; Cell 2005;120:817-829. PMID:15797382