Abstract

The mitochondrial respiratory chain is composed of individual complexes that range widely in terms of size and subunit composition. For example, whereas complex II is approximately 260 kDa and is composed of 4 subunits, complex I is almost 1 MDa and contains 45 different subunits. Furthermore, complexes I, III, IV, and V harbor additional complexity, because their subunits are encoded by both nuclear and mitochondrial DNA. Subunits that are encoded by nuclear genes must be imported into mitochondria before undergoing processing, folding, and assembly with other subunits that are synthesized within the organelle. This process requires the coordinated action of assembly factors with the integration of subunits into intermediate assembly complexes. Recent studies have used various techniques to analyze subunit assembly to gain information into the biogenesis of these respiratory chain complexes and to understand how defects in assembly lead to disease. Here we describe methods to monitor the assembly of newly synthesized subunits encoded by mitochondrial DNA from cultured mammalian cells, as well as the import and assembly of individual subunits encoded by nuclear DNA.