Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 214

As noted above, protein folding can be coupled to protein degradation through the association of chaperones such as ClpA and ClpX with proteases such as ClpP. Complexes such as ClpAP and ClpXP are members of the family of ATP-dependent proteases, which use cleavage of ATP to provide the energy for protein unfolding and proteolysis (see Baker and Sauer, Suggested Reading). These enzymatic machineries are important not only for the destruction of misfolded and denatured proteins, but also for regulated proteolysis to target specific substrate proteins under specific conditions. Their activity can be directed by the presence of a specific degradation tag, a short protein sequence that increases affinity for a specific protease; these tags are similar to the sequence added by tmRNA to target incomplete proteins for degradation (see above). This tag may be present all the time within the target protein, but it becomes available for recognition only under certain conditions, which allows the target protein to be stable under some conditions and unstable under other conditions. Proteolysis can also be controlled by adaptor proteins that deliver specific target proteins to specific proteases. Regulated proteolysis is discussed as a mechanism for gene regulation in chapters 11 and 12.

The Actinobacteria, including important pathogens such as Mycobacterium tuberculosis, utilize a proteasome structure similar to that used in eukaryotes to mediate degradation of specific protein targets. This structure is barrel-shaped and carries out both ATP-dependent and ATP-independent proteolysis. Eukaryotes use a specific protein tag called ubiquitination to direct protein substrates to the proteasome; in Actinobacteria, this is replaced by a different protein modification called pupylation (see Becker and Darwin, Suggested Reading).