Mitochondrial dynamics rely on DRP1-driven fission, while DRP1 requires adaptors MFF, MID49, and MID51 for membrane recruitment. However, key questions remain unresolved: why does MID49/51 overexpression paradoxically inhibit fission? What is the in situ architecture of functional DRP1 assemblies?

To address these issues, a research team led by Xu Tao and Ji Wei at the Institute of Biophysics, Chinese Academy of Sciences, applied ROSE-3D, a three-dimensional super-resolution imaging technique, to directly visualize and characterize the helical assembly of DRP1 complexes in cells. Their findings are published in Protein & Cell.

Key findings from the study include:

Distinct roles of adaptor proteins: MFF knockout caused mitochondrial elongation, whereas MID49/51 single knockout had no significant effect, indicating redundancy. Overexpression of MFF promoted fragmentation, but MID49/51 overexpression caused elongation. This paradox arises because MID49/51 excessively recruit DRP1, leading to “futile sequestration” and insufficient DRP1 at actual fission sites, confirmed by dual-color imaging showing enlarged DRP1 puncta.

Direct visualization of DRP1 helices in cells: Using ROSE-3D, the team resolved helical DRP1 complexes with measured radii of 200–400 nm, a mean pitch of ~160 nm, axial length <400 nm, and typically fewer than 2.5 turns. Quantitative analysis revealed a strong positive correlation between axial length and pitch, suggesting progressive helical contraction during fission, akin to a spring being tightened.

Competitive self-regulatory model for DRP1 assembly: MID49/51 initially recruit DRP1 into dynamic primary assemblies on the outer mitochondrial membrane. When MFF is available, it drives the transition to contracted higher-order structures with reduced pitch and axial length, culminating in membrane constriction and fission. MID49/51 overexpression causes DRP1 sequestration due to relative MFF insufficiency, leading to elongation, whereas MFF overexpression drives rapid fission with smaller, shorter-lived DRP1 assemblies.

This study provides direct in-cell evidence of DRP1 helical assemblies and reveals how distinct adaptor proteins orchestrate mitochondrial fission through a competitive self-regulatory mechanism, offering new molecular insights of mitochondrial fission.
ROSE-3D reveals that adaptor proteins recruits DRP1 to outer mitochondrial membrane to form a diverse helical structure, and different adaptors work together to drive mitochondrial fission.
DOI:10.1093/procel/pwag058