MSF, a novel cytoplasmic chaperone which functions in precursor targeting to mitochondria

Area of Science:

• Mitochondrial biology
• Protein import
• Cellular chaperones

Background:

• Mitochondrial precursor proteins require proper folding for import into mitochondria.
• Aggregation of precursor proteins can lead to import incompetence.
• Cytoplasmic factors may play a role in managing precursor protein folding and import.

Purpose of the Study:

• To elucidate the function of Mitochondrial import stimulation factor (MSF) in mitochondrial protein import.
• To investigate the mechanism by which MSF interacts with mitochondrial precursor proteins and the outer membrane.
• To determine the role of ATP hydrolysis and specific protein modifications in MSF activity.

Main Methods:

• Utilized chemically pure adrenodoxin precursor (pAd) as a model mitochondrial precursor protein.
• Assessed MSF's ability to bind, unfold, and restore import competence to aggregated pAd.
• Measured MSF's ATPase activity in response to precursor proteins and its inhibition by mitochondrial outer membrane (OM) fractions.
• Investigated the effects of trypsin and N-ethylmaleimide (NEM) treatments on MSF and OM interactions.

Main Results:

• MSF unfolds aggregated mitochondrial precursor proteins, preventing loss of import competence and restoring it via ATP hydrolysis.
• Import-incompetent precursors induce MSF ATPase activity, which is inhibited by intact OM but not by trypsin-treated OM (tOM).
• The MSF-precursor complex binds specifically to OM, and this binding is abolished by trypsin treatment of OM or NEM treatment of MSF.

Conclusions:

• MSF functions as a novel cytoplasmic chaperone protein.
• MSF possesses a mitochondrial precursor-targeting function, facilitating import into mitochondria.
• MSF's interaction with the mitochondrial outer membrane is crucial for its targeting activity and is mediated by specific chemical moieties.