2018: Structure of a mitochondrial fission dynamin in the closed conformation
X-ray crystal structures of the mitochondrial fission dynamin DNM1 from the algae Cyanidioschyzon merolae (left) and the bacterial dynamin BDLP1 from the cyanobacteria Nostoc punctiforme (right). Unexpectedly, both are observed in a closed conformation, which has implication for how dynamin-family members couple large scale conformational changes with membrane fission and fusion.
Dynamin 1-like proteins (DNM1-L) are mechanochemical GTPases that induce membrane fission in mitochondria and peroxisomes. They are associated with basic cellular processes such as apoptosis, cell proliferation and mitophagy with implication in many cancers. Their mechanism depends on conformational changes driven by nucleotide and lipid cycling. In this project, we showed the crystal structure of a mitochondrial fission dynamin (CmDnm1) from the algae Cyanidioschyzon merolae. Unlike other eukaryotic dynamin structures, CmDnm1 is in a closed conformation with the GTPase domain compacted against the stalk. This conformation is exciting as it had only been observed previously in dynamin homologues in bacteria. The crystal structure of CmDnm1 now suggests that the closed conformation may be a fundamental conserved property in both membrane fission and fusion dynamins and might be found from bacteria through to humans.
Model for CmDnm1 activation and polymerization on membrane templates.
CmDnm1 also crystallises as a novel diamond-shaped tetramer (above left). To verify the hinge 1 folded CmDnm1 monomer and the tetramer in solution state, we used a suite of cross-linking, photoinduced electron transfer (PET) assays, and negative stain electron microscopy (EM). Our results are important as 1) the closed conformation presents a mechanism for filament collapse and release from the membrane. 2) The CmDnm1 tetramer is suggestive of a mechanism for subunit inactivation and off-membrane storage. 3) The closed conformation raises questions about the completeness of current membrane fission models.
2018: Structural basis for membrane tethering by a bacterial dynamin-like pair
Dynamin-like proteins (DLPs) are large GTPases that restructure membrane. DLPs such as the mitofusins form heterotypic oligomers between isoform pairs that bridge and fuse opposing membranes. In bacteria, heterotypic oligomerisation may also be important for membrane remodelling as most DLP genes are paired within operons. How DLPs tether opposing membranes is unknown. In this project, we show the crystal structure of a DLP heterotypic pair from the pathogen Campylobacter jejuni. A 2:2 stoichiometric tetramer is observed with subunits in a range of different conformations.
The crystal structure of Cj-DLP1/2 tetramer in the apo state. Cj-DLP2 forms a central back-to-back dimer flanked on each side by Cj-DLP1 subunits.
In solution, Cj-DLP1 is connected to Cj-DLP2 by a flexible linker, which suggests a direct mechanism for the long-range binding and bridging of opposing membranes by a bacterial DLP pair. Although the cellular function of Cj-DLP1 and Cj-DLP2 is currently unknown, it is speculated that these proteins may be involved in membrane repair with implication for antibiotic and phage resistance.
Membrane tethering by a bacterial DLP. Cj-DLP1 is connected to Cj-DLP2 by a linker that is likely highly flexible in solution.
Our results also provide mechanistic and structural insights that are relevant to other members of the dynamin superfamily. For example, Cj-DLP1 appears remarkably similar to human Mitofusin 1 and may represent the conformation predicted for Mitofusin 1 to tether membranes.
Cartoon superposition of Cj-DLP1 and Mitofusin 1 partial structure.