AAA+ proteins and substrate recognition, it all depends on their partner in crime

Members of the AAA+ superfamily have been identified in all organisms studied to date. They are involved in a wide range of cellular events. In bacteria, representatives of this superfamily are involved in functions as diverse as transcription and protein degradation and play an important role in the protein quality control network. Often they employ a common mechanism to mediate an ATP-dependent unfolding/disassembly of protein-protein or DNA-protein complexes. In an increasing number of examples it appears that the activities of these AAA+ proteins may be modulated by a group of otherwise unrelated proteins, called adaptor proteins. These usually small proteins specifically modify the substrate recognition of their AAA+ partner protein. The occurrence of such adaptor proteins are widespread; representatives have been identified not only in Escherichia coli but also in Bacillus subtilis, not to mention yeast and other eukaryotic organisms. Interestingly, from the currently known examples, it appears that the N domain of AAA+ proteins (the most divergent region of the protein within the family) provides a common platform for the recognition of these diverse adaptor proteins. Finally, the use of adaptor proteins to modulate AAA+ activity is, in some cases, an elegant way to redirect the activity of an AAA+ protein towards a particular substrate without necessarily affecting other activities of that AAA+ protein while, in other cases, the adaptor protein triggers a complete switch in AAA+ activity.