A mitochondrial insertion directs substrate selection and engagement by ClpX

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Biology

Background:

• Protein unfoldases, like ClpX, utilize AAA+ ATPase domains for function.
• Accessory domains modulate substrate specificity and interactions.
• Mitochondrial ClpX homologs possess a unique insertion (MI) absent in bacterial counterparts.

Purpose of the Study:

• To investigate the role of the mitochondrial insertion (MI) in the function of mitochondrial ClpX.
• To determine if the MI directs interactions with mitochondrial substrates, specifically ALAS.
• To elucidate the MI's contribution to substrate recruitment, activation, and processing.

Main Methods:

• Site-directed mutagenesis to assess the impact of MI truncation and specific mutations.
• Enzyme activity assays to measure ATPase activity and substrate activation.
• In vitro degradation assays using model substrates like casein and ALAS.

Main Results:

• The MI is essential for the recruitment and activation of ALAS by yeast ClpX.
• The MI's role in ALAS degradation by human CLPXP is context-dependent (adaptor-mediated vs. independent).
• MI truncation moderately affects ATPase activity but can be uncoupled from substrate activation efficiency.

Conclusions:

• The MI is a key functional domain in mitochondrial ClpX, facilitating substrate recruitment and accelerating AAA+ motor processing.
• The MI plays a critical role in the efficient degradation of specific mitochondrial substrates like ALAS.
• Understanding the MI's function provides insights into mitochondrial protein quality control and homeostasis.