Cloning and characterization of the mouse Arht2 gene which encodes a putative atypical GTPase

EMB2473/MIRO1, an Arabidopsis Miro GTPase, is required for embryogenesis and influences mitochondrial morphology in pollen

The regulation of mitochondrial biogenesis, subcellular distribution, morphology, and metabolism are essential for all aspects of plant growth and development. However, the molecular mechanisms involved are still unclear. Here, we describe an analysis of the three Arabidopsis thaliana orthologs of the evolutionarily conserved Miro GTPases. Two of the genes, MIRO1 and MIRO2, are transcribed ubiquitously throughout the plant tissues, and their gene products localize to mitochondria via their C-terminal transmembrane domains. While insertional mutations in the MIRO2 gene do not have any visible impact on plant development, an insertional mutation in the MIRO1 gene is lethal during embryogenesis at the zygote to four-terminal-cell embryo stage. It also substantially impairs pollen germination and tube growth. Laser confocal and transmission electron microscopy revealed that the miro1 mutant pollen exhibits abnormally enlarged or tube-like mitochondrial morphology, leading to the disruption of continuous streaming of mitochondria in the growing pollen tube. Our findings suggest that mitochondrial morphology is influenced by MIRO1 and plays a vital role during embryogenesis and pollen tube growth.

The atypical Rho GTPases Miro-1 and Miro-2 have essential roles in mitochondrial trafficking

We recently described the atypical Rho GTPases Miro-1 and Miro-2. These proteins have tandem GTP-binding domains separated by a linker region with putative calcium-binding motives. In addition, the Miro GTPases have a C-terminal transmembrane domain, which confers targeting to the mitochondria. It was reported previously that a constitutively active mutant of Miro-1 induced a clustering of the mitochondria. This response can be separated into two distinct phenotypes: a formation of aggregated mitochondria and the appearance of thread-like mitochondria probably caused by defects in mitochondrial trafficking. The first GTPase domain is required for the clustering of the mitochondria, but the effect is not dependent on the EF-hands. Miro-2 only induces aggregation and not the formation of thread-like mitochondria. Moreover, we show that Miro interacts with the Kinesin-binding proteins, GRIF-1 and OIP106, suggesting that the Miro GTPases form a link between the mitochondria and the trafficking apparatus of the microtubules.

Identification of a new RTN3 transcript, RTN3-A1, and its distribution in adult mouse brain

The Reticulon (RTN) family of proteins is thought to play important roles in the regulation of neuronal regeneration. In this study, we have identified a novel alternative splicing isoform of the RTN gene family, RTN3-A1, which contains an additional 2.3-kb exon. The transcripts of human and mouse RTN3-A1 (about 5.0 kb) were first discovered by database sequence mining and analysis, and verified by cloning and sequencing. Northern blot analysis of 16 human tissues with a common probe of RTN3 transcripts and a specific probe for RTN3-A1 demonstrated that human RTN3-A1 is expressed mainly in brain tissues with a weak expression in the skeletal muscle. With Western blot analysis, the expected 100-kDa RTN3-A1 protein was detected in mouse brain. In situ hybridization with a mouse RTN3-A1-specific cRNA probe revealed that the mouse RTN3-A1 mRNA was regionally expressed in the neurons of the cerebral cortex, hippocampus, hypothalamus, and cerebellum of the adult mouse brain. In contrast to the transcripts of RTN1 and RTN2, RTN3-A1 shares some significant similarity with RTN4-A in exon structure, tissue distribution, and brain expression profile. Since other reports have shown that RTN4-A inhibits neuronal outgrowth and restricts the plasticity of the central nervous system, we speculate that RTN3-A1 might play certain roles in the central nervous system.