Understanding and harnessing energy-dependent proteolysis for controlled protein degradation in bacteria

Abstract

Regulated intracellular protein degradation is critical for cellular viability. In many organisms, degradation controls cell-cycle progression, executes responses to stress-inducing environmental changes, and enables the rapid depletion of unwanted or deleterious proteins. In bacteria, most processive protein degradation is carried out by a family of AAA+ compartmentalized proteases. These molecular machines convert the chemical energy of ATP binding and hydrolysis into mechanical work, forcefully unfolding their substrates as a prelude to proteolysis. The AAA+ ClpXP protease, recognizes short peptide tags (degrons) in substrate proteins either directly or with the aid of dedicated specificity factors (adaptors). The prior identification and detailed biochemical characterization of an efficient ClpXP degron (the ssrA tag) and cognate adaptor (SspB) serve as powerful tools and enable the mechanistic studies presented here. In Chapter 2, I describe a collaborative investigation of substrate denaturation and degradation by ClpXP with single-molecule resolution. Detailed kinetic analysis of these experiments revealed homogenous protease activity across the population of enzymes with comparable levels of microscopic and macroscopic ClpXP activity. These experiments required the development of methods to attach ClpXP to surfaces and stabilize the multimeric enzyme at sub-nanomolar concentrations, advances that should be applicable to future single-molecule studies of complex protein machines. Subsequent chapters describe the development of molecular tools that harness our understanding of targeted proteolysis and enable small-molecule control of degradation. By engineering synthetic substrates, adaptors and proteases, I directly test models previously proposed to explain adaptor function and identify the minimal requirements for adaptor-mediated substrate delivery. Many different configurations of protease and adaptor domains lead to efficient, predictable substrate degradation and demonstrate the highly modular nature of this system. These tools allow for facile, small-molecule controlled protein degradation in vivo and should be valuable in basic research and biotechnology. I also describe a family of synthetic insulated promoters that allow predictable, context-independent levels of protein synthesis.