Interaction between the fission yeast nim1/cdr1 protein kinase and a dynamin-related protein

Bot1p is required for mitochondrial translation, respiratory function, and normal cell morphology in the fission yeast Schizosaccharomyces pombe

Maintenance of cell morphology is essential for normal cell function. For eukaryotic cells, a growing body of recent evidence highlights a close interdependence between mitochondrial function, the cytoskeleton, and cell cycle control mechanisms; however, the molecular details of this interconnection are still not completely understood. We have identified a novel protein, Bot1p, in the fission yeast Schizosaccharomyces pombe. The bot1 gene is essential for cell viability. bot1Delta mutant cells expressing lower levels of Bot1p display altered cell size and cell morphology and a disrupted actin cytoskeleton. Bot1p localizes to the mitochondria in live cells and cofractionates with purified mitochondrial ribosomes. Reduced levels of Bot1p lead to mitochondrial fragmentation, decreased mitochondrial protein translation, and a corresponding decrease in cell respiration. Overexpression of Bot1p results in cell cycle delay, with increased cell size and cell length and enhanced cell respiration rate. Our results show that Bot1p has a novel function in the control of cell respiration by acting on the mitochondrial protein synthesis machinery. Our observations also indicate that in fission yeast, alterations of mitochondrial function are linked to changes in cell cycle and cell morphology control mechanisms.

hob1+, the fission yeast homolog of Bin1, is dispensable for endocytosis or actin organization, but required for the response to starvation or genotoxic stress

BAR (Bin/Amphiphysin/Rvs) adapter proteins have been suggested to regulate endocytosis, actin organization, apoptosis, and transcription, but their precise roles are obscure. There are at least five mammalian genes that encode BAR adapter proteins, including the evolutionarily conserved and ubiquitously expressed Bin1/Amphiphysin-II and Bin3 genes. Bin1 holds special interest as certain splice isoforms localize to the nucleus, interact with the c-Abl and c-Myc oncoproteins, and display tumor suppressor properties. To obtain functional insights, we embarked upon a genetic analysis of the two BAR adapter proteins expressed in the fission yeast Schizosaccharomyces pombe. In a previous work, a role in actin organization and cytokinesis was identified for the Bin3 homolog hob3+. In this study, a role in stress signaling was defined for the Bin1 homolog, hob1+. Notably, hob1+ was dispensable for endocytosis, actin organization, or osmotic sensitivity. Instead, mutation of hob1+ led to slight cell elongation and faulty cell cycle arrest upon nutrient starvation. These defects were complemented by Bin1, but not by Amphiphysin-I, arguing that these genes have distinct functions despite their structural similarity. hob1 delta mutant cells were also hypersensitive to genotoxic stress. This was not related to a faulty checkpoint response, but mutation in the checkpoint gene rad3(+) further exacerbated the sensitivity of hob1 delta mutant cells. Interestingly, mutation of the cell cycle regulator wee1+ partially relieved the sensitivity defect, suggesting that hob1+ may influence the efficiency of DNA repair or checkpoint release after DNA damage. Genetic and biochemical evidence indicated that hob3+ is epistatic to hob1+ in the response to genotoxic stress. Our findings indicate that the Bin1 homolog hob1+ participates in DNA damage signaling and they suggest a novel role for BAR adapter proteins in stress response processes.

Dynamin family of mechanoenzymes

The dynamin family of proteins is continually growing, and in recent years members have been localized to areas of mitochondrial fission, plant phragmoplasts and chloroplasts, and viral ribonucleoprotein complexes. All the dynamin-like proteins examined to-date appear to assemble into oligomers, such as rings or spirals; however, it remains to be determined if a global mechanism of action exists. Even the role of dynamin in vesicle formation remains controversial as to whether it behaves as a molecular switch or as a mechanochemical enzyme.

Fission yeast Msp1 is a mitochondrial dynamin-related protein

We recently identified Msp1p, a fission yeast Schizosaccharomyces pombe dynamin-related protein, which is essential for the maintenance of mitochondrial DNA. The Msp1p sequence displays typical features of a mitochondrial protein. Here we report in vitro and in vivo data that validate that prediction. We demonstrate that the targeting sequence of Msp1p is processed by recombinant mitochondrial processing peptidase and that Msp1p is imported into S. pombe mitochondria in vitro in the presence of cellular extracts. We show that the first 109 residues of Msp1p encompass a functional peptide signal that is sufficient to direct chimera to mitochondria. Immunofluorescence studies indicate that Msp1p staining colocalises with a mitochondrial marker and electron microscopy shows that the protein is located inside the mitochondria. Mitochondrial enrichment and fractionation further confirm that localisation and show that Msp1p is anchored to the matrix side of the mitochondrial inner membrane. Finally, we report that overexpression of the Msp1 protein results in gross alteration of the mitochondrial structure and function. All together our results suggest that Msp1p is an essential component for mitochondrial maintenance.