Tag Recycling in the Pup-Proteasome System is Essential for Mycobacterium smegmatis Survival Under Starvation Conditions.

Many bacteria possess proteasomes and a tagging system that is functionally analogous to the ubiquitin system. In this system, Pup, the tagging protein, marks protein targets for proteasomal degradation. Despite the analogy to the ubiquitin system, where the ubiquitin tag is recycled, it remained unclear whether Pup is similarly recycled, given how the bacterial proteasome does not include a depupylase. We previously showed in vitro that as Pup lacks effective proteasome degradation sites, it is released from the proteasome following target degradation, remaining conjugated to a degradation fragment that can be later depupylated. Here, we tested this model in Mycobacterium smegmatis, using a Pup mutant that is effectively degraded by the proteasome. Our findings indicate that Pup recycling not only occurs in vivo but is also essential to maintain normal pupylome levels and to support bacterial survival under starvation conditions. Accordingly, Pup recycling is an essential process in the mycobacterial Pup-proteasome system.