The Pam18/Tim14–Pam16/Tim16 complex of the mitochondrial translocation motor: The formation of a stable complex from marginally stable proteins

Abstract

The vast majority of mitochondrial proteins are imported from the cytosol. For matrix‐localized proteins, the final step of translocation across the inner membrane is mediated by the mitochondrial translocation motor, of which mhsp70 is a key component. The ATP‐dependent function of mhsp70 is regulated by a complex, composed of a J‐protein (called Pam18 or Tim14) and a J‐like protein (called Pam16 or Tim16), and the nucleotide exchange factor Mge1. In this study, we investigated the structural properties of a recombinant purified Pam18/Tim14–Pam16/Tim16 complex using cross‐linking with the bifunctional reagent DSS and CD‐spectroscopy. The results of the study show that both Pam18/Tim14 and Pam16/Tim16 are thermally unstable proteins that unfold at very low temperatures (T m values of 16.5°C and 29°C, respectively). Upon mixing the proteins in vitro, or when both proteins are co‐overexpressed in bacteria, Pam18/Tim14 and Pam16/Tim16 form a heterodimer that is thermally more stable than the individual proteins (T m = 41°C). Analysis of the properties of the complex in GdnHCl shows that dissociation of the heterodimer is the limiting step in achieving full denaturation.