The Mitochondrial Import Complex MIM Functions as Main Translocase for α-Helical Outer Membrane Proteins

Area of Science:

• Mitochondrial biology
• Protein import and biogenesis
• Cellular membrane dynamics

Background:

• The mitochondrial outer membrane hosts integral proteins with α-helical or β-barrel structures.
• While the import of β-barrel and multi-spanning α-helical proteins is understood, single-spanning protein biogenesis remains unclear.
• Single-spanning proteins represent over half of integral outer membrane proteins.

Purpose of the Study:

• To elucidate the biogenesis mechanism of single-spanning proteins in the mitochondrial outer membrane.
• To investigate the role and mechanisms of the mitochondrial import (MIM) complex in protein insertion.

Main Methods:

• Investigated the function of the yeast MIM complex in protein insertion.
• Characterized the dynamic populations and interactions of the MIM complex.
• Examined MIM complex interactions with the translocase of the outer membrane (TOM) and sorting and assembly machinery (SAM) complexes.

Main Results:

• The MIM complex facilitates the insertion of proteins with N-terminal (signal-anchored) or C-terminal (tail-anchored) membrane anchors.
• MIM exists in three dynamic populations, interacting with TOM for receptor-mediated import and independently for signal-anchored proteins.
• MIM and SAM coupling is crucial for early assembly steps of TOM subunits.

Conclusions:

• The MIM complex is a key player in the insertion of single-spanning proteins into the mitochondrial outer membrane.
• MIM demonstrates versatility, acting as a major protein translocase for various protein types.
• This study clarifies a significant gap in understanding mitochondrial protein biogenesis.