The mitochondrial outer membrane protein assembly machinery of the human fungal pathogen Candida albicans

Abstract

Mitochondria are essential organelles with protein complexes embedded in their membranes. These act as molecular machines that produce energy and carefully regulate what goes in to and out of the organelle. The outer membrane machinery provides the interface between the mitochondrion and the rest of the cell and coordinates the activity of each mitochondrion in keeping with the metabolic requirements of the cell. Research over the last twenty years has identified many of the components and complexes that assemble this membrane machinery, but many questions remain about how these machines function and how they are assembled. While we have learnt a great deal about these molecular machines in the tractable model system Saccharomyces cerevisiae, its unusual regulation of mitochondrial function makes it difficult to glean general principles from discoveries in this organism alone. By comparing findings in other organisms we can determine which features are conserved and learn more about the evolutionary origins of the molecular components. The work in this thesis focuses on the protein import machinery of the mitochondrial outer membrane, in particular the sorting and assembly machinery (SAM) complex. My thesis is the first in-depth study of the mitochondrial import machinery of the human fungal pathogen Candida albicans. I outline the mitochondrial protein import pathways with a focus on the roles and partnerships of the SAM complex. Examining the import pathways in C. albicans in comparison to S. cerevisiae revealed interesting examples of rewiring of the protein import pathways into the intermembrane space. These preliminary studies also show that mitochondria from C. albicans more efficiently assembles proteins into the mitochondrial outer membrane, which leads to a more detailed investigation of the composition and behaviour of the SAM complex. This reveals new roles for the SAM complex components; Sam37, Sam35 and Mdm10 and a new component of the C. albicans SAM complex, Sam51. The work presented here describes the functions and interactions of C. albicans SAM complex components, a detailed characterisation of their roles in the import and assembly of outer membrane proteins, and the partnerships between the C. albicans SAM complex and other outer membrane protein complexes.