The heme molecule plays an important role in the electron transfer performed by redox proteins such as cytochromes. In addition, it has been suggested that heme has a structural role in protein folding and assembly. However, it is now known that protein folding and assembly do not occur spontaneously in vivo, but rather, a series of helper proteins, known as molecular chaperones, are required. Thus, in this article, we show the role of heme and various molecular chaperones in the in vivo assembly of a tetrameric membrane protein called succinate-ubiquinone reductase (SQR, complex II), which contains heme b and functions as a dehydrogenase in the aerobic respiratory chain, in Escherichia coli.