The dynamin family of proteins consists of unique GTPases involved in membrane fission and fusion events throughout the cell. Our goal is to understand the dynamic structural properties of these proteins and correlate them with their diverse cellular functions. Dynamin is essential for endocytosis and vesiculation events in the cell. Additional dynamin family members have been implicated in a variety of fundamental cellular processes, including mitochondrial fission and fusion, anti-viral activity, cell plate formation and chloroplast biogenesis. Among these proteins, self-assembly and oligomerization into ordered structures is a common characteristic and, for the majority, is essential for their function. Although there is a wealth of information regarding dynamin, little is known about the structural properties of dynamin-related proteins. To determine if a common mechanism of action exists among the dynamin family members, we examined the structure and function of Dnm1, a yeast dynamin family member involved in mitochondria fission and Opa1, a human dynamin involved in mitochondria fusion. Dnm1 Previously, in collaboration with Dr. Jodi Nunnari from UC Davis, we have shown that Dnm1 assembles into large spirals, 100 nm in diameter compared to the 50 nm for dynamin spirals. Remarkably, the diameter of Dnm1 spirals is the same as that of mitochondrial constriction sites observed in cells. Dnm1 also assembles onto liposomes in the absence or presence of nucleotides, forming well-decorated tubes. In addition, the GTP hydrolysis rate of Dnm1 is highly cooperative with respect to its self-assembly state and concentration, which is consistent with the kinetic properties of dynamin. These results suggest that although dynamin family members share common characteristics, their structural properties are uniquely tailored to fit their function. Though Dnm1 can assemble onto liposomes in vitro, their assembly in cells is tightly regulated. Two additional mitochondrial proteins, Fis1 and Mdv1, are required and function together with Dnm1 in mitochondrial division. We have shown that Mdv1 interacts with Dnm1 only when Dnm1 is assembled into GTP-bound ring or spiral structures. GTPase mutants defective in binding GTP, which failed to self-assemble into spirals, no longer localized with Mdv1. These findings suggest Mdv1 functions in fission by stabilizing or promoting the formation of Dnm1 into spiral-like structures. Mdv1 may accomplish this by stabilizing the GTP bound form of Dnm1 or by acting as a nucleator, promoting Dnm1 to form spirals at sites of membrane constriction. Recently, we have solved the structure of Dnm1 bound to lipid using the IHRSR method and found the helical parameters significantly different than dynamin. In the 3D map of Dnm1 there are 24 repeating subunits per turn of the helix and the repeating subunit consists of a tetramer (96 Dnm1 molecules per turn). In addition, we have shown that upon GTP addition the Dnm1-lipid tubes constrict in diameter from 120 nm to 70 nm. The overall structure reveals a loose association with the underlying lipid bilayer, which supports the model of a highly flexible helix that is capable of undergoing a large conformational change. OPA1 Previously, in collaboration with Dr. David Chan from Cal Tech, we examined the structure of OPA1 by negative stain and cryo electron microscopy. Mutations in OPA1 (autosomal dominant optic atrophy) can lead to an inherited neuropathy of the retinal ganglion cells. In the cell, OPA1 has been shown to be essential for the fusion of the inner mitochondrial membranes, but its mechanism of action remains poorly understood. Addition of OPA1 to liposomes containing cardiolipin results in enhanced GTP hydrolysis rate and promotes OPA1 to self-assemble into helical arrays around the lipid, forming protein-lipid tubes. Currently we are solving the structure of OPA1 by helical and single particle image processing methods. The conformational state of OPA1 during GTP hydrolysis is also being examined.
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|Sundborger, Anna; Soderblom, Cynthia; Vorontsova, Olga et al. (2011) An endophilin-dynamin complex promotes budding of clathrin-coated vesicles during synaptic vesicle recycling. J Cell Sci 124:133-43|
|Ban, Tadato; Heymann, Jurgen A W; Song, Zhiyin et al. (2010) OPA1 disease alleles causing dominant optic atrophy have defects in cardiolipin-stimulated GTP hydrolysis and membrane tubulation. Hum Mol Genet 19:2113-22|