The core MICOS complex subunit Mic60 has been substituted by two cryptic mitofilin-containing proteins in Euglenozoa

Cristae enclose respiratory chain protein complexes, making them the bioenergetic subcompartments of mitochondria. The MICOS complex is among the inducers of sharp membrane curvature needed for crista formation. Resembling the respiratory chain complexes, MICOS is organized around a core protein, the mitofilin-domain bearing Mic60, that was inherited from the alphaproteobacterial progenitor of mitochondria. Extant alphaproteobacteria express Mic60 to form their own bioenergetic subcompartments, demonstrating the permeance of Mic60’s form and function during the long span of prokaryotic and eukaryotic evolution. Yet, unlike virtually all aerobic eukaryotes, Mic60 is not encoded in any genome of the multifarious protists that comprise the phylum Euglenozoa, including trypanosomes. Here, we show that Mic60 has been replaced in euglenozoans by two cryptic mitofilin domain-containing MICOS subunits, Mic34 and Mic40. Sharply contrasting Mic60 from two domains of life, these are not integral membrane proteins. Reverse genetics revealed they are intertwined with the oxidative protein folding pathway required for mitochondrial–and crista–biogenesis, veiling a potential membrane remodeling role. Nevertheless, Mic34 binds to phospholipid bilayers in vitro . Mic34 and Mic40 heterologous expression remodels gammaproteobacterial cytoplasmic membranes, like prokaryotic and eukaryotic Mic60. Astoundingly, Mic34 overexpression elaborates the simplified tubular mitochondrion of a Trypanosoma brucei life cycle stage with repressed oxidative phosphorylation. The structural correspondence of Mic34’s mitofilin domain to Mic60’s predicted mutations to conserved motifs that ablated Mic34’s mitochondria-remodeling activity. Thus, the mitofilin protein family is more diverse than originally supposed, with two of its structurally most divergent members altering the core of euglenozoan MICOS.