Interchangeable adaptors regulate mitochondrial dynamin assembly for membrane scission

Abstract

Significance Mitochondrial fission is critical for mammalian cell division, mitophagy, and development. Fission initiates via recruitment of dynamin-related GTPases to the mitochondrial surface. In yeast and human, the recruitment utilizes adaptors that differ in sequence and predicted structure. Key unresolved issues are whether these adaptors function independently in membrane recruitment and whether a single adaptor and GTPase are sufficient to catalyze scission. We show that three human adaptors work interchangeably with a single mitochondrial dynamin to accomplish fission. We also show that an adaptor alters the architecture of the dynamin polymer in a manner that could facilitate membrane constriction and severing.

A Targeted Mutation Identified through pK Measurements Indicates a Postrecruitment Role for Fis1 in Yeast Mitochondrial Fission

“…Yeast mitochondrial fission has long been known to depend on Fis1 as a membrane-tethered anchor for recruiting Mdv1 and Dnm1 (8,44,46,47), but the idea that Fis1 acts only as an anchor without further contribution to the mechanism of membrane scission has gained favor with the discovery of other natural anchors (49 -51) and with the demonstration that artificially tethering the C-terminal portion of Mdv1 to mitochondria can partially rescue morphology in yeast cells that lack Fis1, Mdv1, and Caf4 (13). A nominal role for Fis1 in Dnm1 assembly and hydrolysis was shown with recombinant proteins (36), although these studies involved Fis1 lacking its transmembrane domain that when present would dramatically increase the effective concentration at the membrane.…”

“…In sequential models, Mdv1 acts as an obligate adaptor between Fis1 and Dnm1, but no mammalian Mdv1 ortholog has been identified, suggesting that mammalian and yeast fission mechanisms are divergent. This view is supported by a potentially divergent role for Fis1 in mitophagy (9,52,53) and the presence of non-fungal tail anchors (Mff and Mid-49/51) capable of mammalian Dnm1 (Drp1) recruitment independent of Fis1 (13,40,49,51,54,55). However, Fis1 and Dnm1 (Drp1 in mammals) are the only two proteins conserved for mitochondrial fission in all species, and mutating conserved residues in yeast Fis1 disrupts Dnm1 pulldowns, suggesting conserved Fis1/Dnm1 interactions in fission (16).…”

“…Complicating interpretation of these experiments is the presence of Caf4, an Mdv1-like protein that interacts with both Fis1 and Dnm1 but is not essential for fission (11,12) and promotes mitochondrial fission much less effectively than Mdv1 (12). A passive role for Fis1 in this sequential model has been inferred from the ability of artificially tethered Mdv1 to partially support normal mitochondrial morphology in the absence of Fis1 (13). At odds with this model are the observations that Mdv1 arrives at mitochondrial foci after Dnm1 in time lapse experiments, not before (14).…”

Dynamin-related Protein 1 (Drp1) Promotes Structural Intermediates of Membrane Division

“…A number of attempts have also been made in reconstituted systems to unravel the GTPase and membrane remodeling activity of Drp1 (26, 29 -32). For example, Drp1 self-assembly into rings and spirals in the absence of lipids is affected by the presence of MiD49 or MiD51 (31,33). However, the ability of Drp1 to mediate membrane constriction remains under debate because assembly around liposomes and membrane tubulation have mainly been reported for pure phosphatidylserine membranes under the harsh conditions of electron microscopy (26,31).…”

“…For example, Drp1 self-assembly into rings and spirals in the absence of lipids is affected by the presence of MiD49 or MiD51 (31,33). However, the ability of Drp1 to mediate membrane constriction remains under debate because assembly around liposomes and membrane tubulation have mainly been reported for pure phosphatidylserine membranes under the harsh conditions of electron microscopy (26,31). Additional approaches that provide insight into the dynamic aspects of membrane remodeling by Drp1 under near-physiological conditions will be essential to understand the molecular mechanisms behind this process.…”