Trans-targeting of protease substrates by conformationally activating a regulable ClpX-recognition motif.

ABSTRACT

Conversion of bacteriophage Mu repressor to ClpXP-sensitive form correlates with induced local flexibility at the ClpX recognition motif located at the C-terminal end. Changing the C-terminal valine to an alanine (RepV196A) caused the degradation tag to be constitutively active like that of mutant repressors called Vir, which have a dominant ClpXP-sensitive conformation. However, unlike Vir, RepV196A was unable to convert wild-type repressor (Rep) to the ClpXP-sensitive form. In mixtures with Rep, only RepV196A was rapidly degraded by ClpXP. Unlike Rep, RepV196A was ClpXP sensitive without induced C-terminal flexibility. And unlike adaptor proteins that tether and deliver substrates to ClpX for trans-targeting, Vir promoted rapid degradation of Rep by ClpX deleted for the tethering site that binds adaptor proteins. Therefore, Rep's ClpX recognition motif has regulable properties, allowing an alternative trans-targeting mechanism in which an inactive degradation tag is turned on by induced conformational changes.