Structural basis for the nucleotide-dependent dimerization of the large G protein atlastin-1/SPG3A.

The large GTPase atlastin belongs to the dynamin superfamily that has been widely implicated in facilitating membrane tubulation, fission, and in select cases, fusion. Mutations spread across atlastin isoform 1 (atlastin-1) have been identified in patients suffering from hereditary spastic paraplegia (HSP), a neurodegenerative disorder affecting motor neuron function in the lower extremities. On a molecular level, atlastin-1 associates with high membrane curvature and fusion events at the endoplasmic reticulum and cis-Golgi. Here we report crystal structures of atlastin-1 comprising the G and middle domains in two different conformations. Although the orientation of the middle domain relative to the G domain is different in the two structures, both reveal dimeric assemblies with a common, GDP-bound G domain dimer. In contrast, dimer formation in solution is observed only in the presence of GTP and transition state analogs, similar to other G proteins that are activated by nucleotide-dependent dimerization. Analyses of solution scattering data suggest that upon nucleotide binding, the protein adopts a somewhat extended, dimeric conformation that is reminiscent of one of the two crystal structures. These structural studies suggest a model for nucleotide-dependent regulation of atlastin with implications for membrane fusion. This mechanism is affected in several mutants associated with HSP, providing insights into disease pathogenesis.

A novel frameshift mutation of the mtDNA COIII gene leads to impaired assembly of cytochrome c oxidase in a patient affected by Leigh-like syndrome.

We report on a novel frameshift mutation in the mtDNA gene encoding cytochrome c oxidase (COX) subunit III. The proband is an 11-year-old girl with a negative family history and an apparently healthy younger brother. Since 4 years of age, she has developed a progressive spastic paraparesis associated with ophthalmoparesis and moderate mental retardation. The presence of severe lactic acidosis and Leigh-like lesions of putamina prompted us to perform muscle and skin biopsies. In both, a profound, isolated defect of COX was found by histochemical and biochemical assays. Sequence analysis of muscle mtDNA resulted in the identification of a virtually homoplasmic frameshift mutation in the COIII gene, due to the insertion of an extra C at nucleotide position 9537 of mtDNA. Although the 9537C(ins) does not impair transcription of COIII, no full-length COX III protein was detected in mtDNA translation assays in vivo. Western blot analysis of two-dimensional blue-native electrophoresis showed a reduction of specific crossreacting material and the accumulation of early-assembly intermediates of COX, whereas the fully assembled complex was absent. One of these intermediates had an electrophoretic mobility different from those seen in controls, suggesting the presence of a qualitative abnormality of COX assembly. Immunostaining with specific antibodies failed to detect the presence of several smaller subunits in the complex lacking COX III, in spite of the demonstration that these subunits were present in the crude mitochondrial fraction of patient's cultured fibroblasts. Taken together, the data indicate a role for COX III in the incorporation and maintenance of smaller COX subunits within the complex.