Mitochondrial Presequence Translocase: Switching between TOM Tethering and Motor Recruitment Involves Tim21 and Tim17

Abstract

The presequence translocase of the inner mitochondrial membrane (TIM23 complex) operates at a central junction of protein import. It accepts preproteins from the outer membrane TOM complex and directs them to inner membrane insertion or, in cooperation with the presequence translocase-associated motor (PAM), to the matrix. Little is known of how the TIM23 complex coordinates these tasks. We have identified Tim21 (YGR033c) that interacts with the TOM complex. Tim21 is specific for a TIM23 form that cooperates with TOM and promotes inner membrane insertion. Protein translocation into the matrix requires a switch to a Tim21-free, PAM bound presequence translocase. Tim17 is crucial for the switch by performing two separable functions: promotion of inner membrane insertion and binding of Pam18 to form the functional TIM-PAM complex. Thus, the presequence translocase is not a static complex but switches between TOM tethering and PAM binding in a reaction cycle involving Tim21 and Tim17.

The presequence pathway is involved in protein sorting to the mitochondrial outer membrane

“…Import of Om45 was not inhibited in tim21D mitochondria (Fig 3D). Tim50 and Tim23 have receptor functions for presequence-carrying preproteins in the intermembrane space [30][31][32] and promote translocation of the preproteins through the TOM complex [23,29]. Accumulation of Om45-DHFR in the TOM complex depended on these subunits of the presequence translocase in a manner comparable to the accumulation of the cleavable preprotein Oxa1 in TOM (Fig 3E) [29].…”

“…As control, import of Ugo1 and Tom40 were not inhibited or even enhanced in the mutant mitochondria (Fig 3B and C). Tim21 plays a regulatory function in the TIM23 complex, yet is dispensable for import of presequence-carrying preproteins [29]. Import of Om45 was not inhibited in tim21D mitochondria (Fig 3D).…”

“…We thus analysed mutant mitochondria, which are impaired in subunits of the TIM23 complex [29][30][31][32]. Conditional mutants of the essential subunits Tim23, Tim50 and Tim17 were considerably impaired in the import of Om45 (Fig 3B and C).…”

“…Tim50 and Tim23 have receptor functions for presequence-carrying preproteins in the intermembrane space [30][31][32] and promote translocation of the preproteins through the TOM complex [23,29]. Accumulation of Om45-DHFR in the TOM complex depended on these subunits of the presequence translocase in a manner comparable to the accumulation of the cleavable preprotein Oxa1 in TOM (Fig 3E) [29]. Import of Om45 32 A, B [ 35 S]Om45 was imported into WT and mutant mitochondria as described in the legend of Fig 1B and Fig S2E), indicating that the presequence translocase is involved in import of the large C-terminal domain of Om45 into the intermembrane space.…”

Mic60/Mitofilin determines MICOS assembly essential for mitochondrial dynamics and mtDNA nucleoid organization

“…1,2 In contrast, the mitochondrial inner membrane consists of two morphologically distinct regions: the inner boundary membrane is in close proximity to the outer membrane and the cristae membranes that are large tubular invaginations. [3][4][5][6][7][8] The mitochondrial inner boundary and cristae membrane are physically separated by cristae junctions, which are narrow tubular or slot-like structure. 4,9 The mitochondrial cristae are arranged in regular arrays and are the main sites of ATP production in the mitochondria, but the molecules that are associated with the maintenance of cristae architecture still remain elusive.…”