CBC Department Special Seminar: "Learning the Secrets of AAA+ Protease Specificity"

Presenter:

Dr. Alireza Ghanbarpour

Image

Abstract:

Degradation is a vital aspect of the proteostasis network, which orchestrates protein synthesis, folding, and turnover. Following synthesis, some proteins are stable for long periods, whereas others are degraded rapidly to allow new cellular functions or to remove damaged molecules. From bacteria to humans, intracellular degradation is chiefly mediated by ATP-dependent AAA+ proteases, including ClpXP and FtsH, which degrade abnormal proteins and reshape the proteome as cells respond to environmental stress or attacks by pathogens (1-3). Selective degradation depends on initial protease recognition of an unstructured degron sequence, typically at a terminus of a target protein, followed by ATP-powered unfolding of any native structure proximal to the degron and processive translocation of the unfolded polypeptide into a sequestered proteolysis chamber (4, 5).

As degradation is irreversible, rigorous regulation is essential to avoid unintended proteolysis. Achieving such precision in a crowded cell (6, 7) is no small feat. How do AAA+ proteases achieve a high level of specificity but adapt their activities in response to environmental cues or developmental needs? Part of the answer is accessory adaptor proteins, which selectively facilitate or inhibit substrate recognition, but how adaptors function is poorly understood. My research investigates substrate specificity and adaptor function for two AAA+ proteases: membrane-bound FtsH, with special roles in membrane-protein degradation, and ClpXP, a cytoplasmic enzyme, employing a multidisciplinary approach that combines structural biology, in vitro reconstitution, biochemistry, and cellular assays. I will discuss cryo-EM structures and experiments that show that the conformation of the axial channel of ClpX is one determinant of substrate specificity (8), that reveal how an adaptor protein delivers a substrate to ClpX axial channel to enhance specificity (9), and that uncover a large complex of FtsH with additional membrane proteins that form an open cage-like structure around the protease, suggesting a novel model for substrate recognition by membrane anchored AAA proteases.