Exploring the activation mechanism of the Mic60 subcomplex governing mitochondrial crista junctions

Abstract

Crista junctions (CJs) are narrow infoldings of the inner mitochondrial membrane, serving as entry points into the cristae microenvironment. The mitochondrial contact site and cristae organizing system (MICOS) complex is a highly conserved protein machinery controlling CJ biogenesis and architecture, but its assembly mechanism driving CJ formation has remained unclear. By determining a crystal structure of a Chaetomium thermophilum Mic60-Mic19 complex and combining it with modelling and mutagenesis studies, we identify an extended helical region (EHR) of the Mic60 mitofilin domain and a further Mic19 helix as crucial elements of complex formation. In vitro cross-linking data and hydrogen-deuterium exchange mass spectrometry resolve domain assembly and conformational dynamics of the Mic60-Mic19 subcomplex and point to the EHR as a molecular switch controlling Mic60 transition from a closed dimer to a Mic19-bound hetero-octamer. Our data provide a structural model for Mic60 activation and therefore MICOS assembly as a prerequisite for crista junction formation.