Directly from the source: endogenous preparations of molecular machines.

Purification from a source enriched in large macromolecular machines with basic cellular function is still the method of choice in many cases. Such complexes occur in sufficiently high copy numbers in the cell and can be isolated using classical protein purification protocols. Although advanced DNA recombinant technologies and sophisticated overexpression strategies are available, many complexes like the ribosome, RNA polymerase II and membrane protein complexes involved in photosynthesis or in oxidative phosphorylation can only be purified from a rich source. Here, we review recent accomplishments and limitations in applying this strategy.