The N-end rule pathway: from recognition by N-recognins, to destruction by AAA+proteases

Intracellular proteolysis is a tightly regulated process responsible for the targeted removal of unwanted or damaged proteins. The non-lysosomal removal of these proteins is performed by processive enzymes, which belong to the AAA+superfamily, such as the 26S proteasome and Clp proteases. One important protein degradation pathway, that is common to both prokaryotes and eukaryotes, is the N-end rule. In this pathway, proteins bearing a destabilizing amino acid residue at their N-terminus are degraded either by the ClpAP protease in bacteria, such as Escherichia coli or by the ubiquitin proteasome system in the eukaryotic cytoplasm. A suite of enzymes and other molecular components are also required for the successful generation, recognition and delivery of N-end rule substrates to their cognate proteases. In this review we examine the similarities and differences in the N-end rule pathway of bacterial and eukaryotic systems, focusing on the molecular determinants of this pathway.