Fig. 6.

Tim44 and its interaction with the TIM23 translocon. a The multiple interactions of the two domains of Tim44: N-terminal domain (NTD) and C-terminal domain (CTD). In the expanded region of NTD, regions of Pam16 interaction, Tim23 crosslinking, presequence binding, and residue R180, the site of amino acid substitution (ts) that affects interaction of both Tim23 and Hsp70, are indicated. This “hot spot” is a candidate for an important role in Tim44 function, such as initiation activation of motor upon entrance of presequence into the matrix driven by the membrane potential. In the CTD (PDB entry 2FXT) residues at positions that crosslink, when having a photoactivatable amino acid, are shown in sphere representation crosslinked to Tim17 (gray) or Tim23 (orange). b Cartoon of Tim44 interaction with the TIM23 translocon, illustrating the dilemma posed by data indicating that both the NTD and CTD of Tim44 (pink) interact with 24-residue loop 1 of Tim23. This short length likely precludes simultaneous binding due to steric hindrance. Top: As the TIM23 translocon contains at least two Tim23 molecules, one Tim44 could interact with two Tim23 molecules simultaneously. Bottom: Alternatively, the NTD and CTD of Tim44 could toggle back and forth, interacting only with one Tim23 molecule, potentially playing a role in regulating or driving efficiency of the motor. Transmembrane helix 1 and 2 that flank loop 1 of Tim23 are indicated