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The yeast gene, MDM20, is necessary for mitochondrial inheritance and organization of the actin cytoskeleton.

Hermann GJ, King EJ, Shaw JM - J. Cell Biol. (1997)

Bottom Line: Together, these results provide compelling evidence that mitochondrial inheritance in yeast is an actin-mediated process.Although MDM20 and TPM1 are both required for the formation and/or stabilization of actin cables, mutations in these genes disrupt mitochondrial inheritance and nuclear segregation to different extents.Thus, Mdm20p and Tpm1p may act in vivo to establish molecular and functional heterogeneity of the actin cytoskeleton.

View Article: PubMed Central - PubMed

Affiliation: Department of Biology, University of Utah, Salt Lake City 84112, USA.

ABSTRACT
In Saccharomyces cerevisiae, the growing bud inherits a portion of the mitochondrial network from the mother cell soon after it emerges. Although this polarized transport of mitochondria is thought to require functions of the cytoskeleton, there are conflicting reports concerning the nature of the cytoskeletal element involved. Here we report the isolation of a yeast mutant, mdm20, in which both mitochondrial inheritance and actin cables (bundles of actin filaments) are disrupted. The MDM20 gene encodes a 93-kD polypeptide with no homology to other characterized proteins. Extra copies of TPM1, a gene encoding the actin filament-binding protein tropomyosin, suppress mitochondrial inheritance defects and partially restore actin cables in mdm20 delta cells. Synthetic lethality is also observed between mdm20 and tpm1 mutant strains. Overexpression of a second yeast tropomyosin, Tpm2p, rescues mutant phenotypes in the mdm20 strain to a lesser extent. Together, these results provide compelling evidence that mitochondrial inheritance in yeast is an actin-mediated process. MDM20 and TPM1 also exhibit the same pattern of genetic interactions; mutations in MDM20 are synthetically lethal with mutations in BEM2 and MYO2 but not SAC6. Although MDM20 and TPM1 are both required for the formation and/or stabilization of actin cables, mutations in these genes disrupt mitochondrial inheritance and nuclear segregation to different extents. Thus, Mdm20p and Tpm1p may act in vivo to establish molecular and functional heterogeneity of the actin cytoskeleton.

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Actin cables lacking in mdm20Δ cells are partially  restored by introducing extra copies of TPM1 and TPM2.  MDM20+ cells (JSY999) carrying pRS426 (A–D) and mdm20Δ  cells (JSY1065) carrying pRS426 (E–H), pRS426-TPM1 (I–L), or  GAL1-TPM2 (M–P) were grown in SD − Ura or SGal − Ura at  25°C, and then shifted to 25° and 37°C for 3 h. Cells were stained  with rhodamine-phalloidin and visualized by DIC or fluorescence  microscopy as indicated. Representative cells are shown. Bar, 5 μm.
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Figure 5: Actin cables lacking in mdm20Δ cells are partially restored by introducing extra copies of TPM1 and TPM2. MDM20+ cells (JSY999) carrying pRS426 (A–D) and mdm20Δ cells (JSY1065) carrying pRS426 (E–H), pRS426-TPM1 (I–L), or GAL1-TPM2 (M–P) were grown in SD − Ura or SGal − Ura at 25°C, and then shifted to 25° and 37°C for 3 h. Cells were stained with rhodamine-phalloidin and visualized by DIC or fluorescence microscopy as indicated. Representative cells are shown. Bar, 5 μm.

Mentions: Taken together, the data presented above suggested that (a) MDM20 and TPM1 participate in overlapping, essential cellular functions; and (b) Tpm2p can partially substitute for Mdm20p when overexpressed. Given what is known about the function of tropomyosins (Pittenger et al., 1994), these results also suggested that the integrity of the actin cytoskeleton is important for mitochondrial inheritance during yeast budding, and that the mdm20 mutation affects the assembly or organization of F-actin. To further investigate this possibility, we examined the actin cytoskeleton in mdm20 cells by rhodamine-phalloidin staining. At both 25° and 37°C, wild-type cells contained well- defined actin cables and cortical actin patches concentrated in growing buds (Fig. 5, A–D). In contrast, actin cables were absent in 100% of the mdm20 cells examined at both temperatures (Fig. 5, E–H). mdm20Δ cells were able to divide, however, and contained brightly staining cortical patches that were predominantly clustered in buds at 25°C (Fig. 5, E and F; >95% of cells) and partially delocalized at 37°C (Fig. 5, G and H; >50% of cells). The fact that mitochondrial inheritance defects are already present at 25°C in the mdm20 mutant (at a temperature where patches are still properly localized) suggests that it is the disruption of actin cables that is primarily responsible for the lack of mitochondrial transmission.


The yeast gene, MDM20, is necessary for mitochondrial inheritance and organization of the actin cytoskeleton.

Hermann GJ, King EJ, Shaw JM - J. Cell Biol. (1997)

Actin cables lacking in mdm20Δ cells are partially  restored by introducing extra copies of TPM1 and TPM2.  MDM20+ cells (JSY999) carrying pRS426 (A–D) and mdm20Δ  cells (JSY1065) carrying pRS426 (E–H), pRS426-TPM1 (I–L), or  GAL1-TPM2 (M–P) were grown in SD − Ura or SGal − Ura at  25°C, and then shifted to 25° and 37°C for 3 h. Cells were stained  with rhodamine-phalloidin and visualized by DIC or fluorescence  microscopy as indicated. Representative cells are shown. Bar, 5 μm.
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Related In: Results  -  Collection

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Figure 5: Actin cables lacking in mdm20Δ cells are partially restored by introducing extra copies of TPM1 and TPM2. MDM20+ cells (JSY999) carrying pRS426 (A–D) and mdm20Δ cells (JSY1065) carrying pRS426 (E–H), pRS426-TPM1 (I–L), or GAL1-TPM2 (M–P) were grown in SD − Ura or SGal − Ura at 25°C, and then shifted to 25° and 37°C for 3 h. Cells were stained with rhodamine-phalloidin and visualized by DIC or fluorescence microscopy as indicated. Representative cells are shown. Bar, 5 μm.
Mentions: Taken together, the data presented above suggested that (a) MDM20 and TPM1 participate in overlapping, essential cellular functions; and (b) Tpm2p can partially substitute for Mdm20p when overexpressed. Given what is known about the function of tropomyosins (Pittenger et al., 1994), these results also suggested that the integrity of the actin cytoskeleton is important for mitochondrial inheritance during yeast budding, and that the mdm20 mutation affects the assembly or organization of F-actin. To further investigate this possibility, we examined the actin cytoskeleton in mdm20 cells by rhodamine-phalloidin staining. At both 25° and 37°C, wild-type cells contained well- defined actin cables and cortical actin patches concentrated in growing buds (Fig. 5, A–D). In contrast, actin cables were absent in 100% of the mdm20 cells examined at both temperatures (Fig. 5, E–H). mdm20Δ cells were able to divide, however, and contained brightly staining cortical patches that were predominantly clustered in buds at 25°C (Fig. 5, E and F; >95% of cells) and partially delocalized at 37°C (Fig. 5, G and H; >50% of cells). The fact that mitochondrial inheritance defects are already present at 25°C in the mdm20 mutant (at a temperature where patches are still properly localized) suggests that it is the disruption of actin cables that is primarily responsible for the lack of mitochondrial transmission.

Bottom Line: Together, these results provide compelling evidence that mitochondrial inheritance in yeast is an actin-mediated process.Although MDM20 and TPM1 are both required for the formation and/or stabilization of actin cables, mutations in these genes disrupt mitochondrial inheritance and nuclear segregation to different extents.Thus, Mdm20p and Tpm1p may act in vivo to establish molecular and functional heterogeneity of the actin cytoskeleton.

View Article: PubMed Central - PubMed

Affiliation: Department of Biology, University of Utah, Salt Lake City 84112, USA.

ABSTRACT
In Saccharomyces cerevisiae, the growing bud inherits a portion of the mitochondrial network from the mother cell soon after it emerges. Although this polarized transport of mitochondria is thought to require functions of the cytoskeleton, there are conflicting reports concerning the nature of the cytoskeletal element involved. Here we report the isolation of a yeast mutant, mdm20, in which both mitochondrial inheritance and actin cables (bundles of actin filaments) are disrupted. The MDM20 gene encodes a 93-kD polypeptide with no homology to other characterized proteins. Extra copies of TPM1, a gene encoding the actin filament-binding protein tropomyosin, suppress mitochondrial inheritance defects and partially restore actin cables in mdm20 delta cells. Synthetic lethality is also observed between mdm20 and tpm1 mutant strains. Overexpression of a second yeast tropomyosin, Tpm2p, rescues mutant phenotypes in the mdm20 strain to a lesser extent. Together, these results provide compelling evidence that mitochondrial inheritance in yeast is an actin-mediated process. MDM20 and TPM1 also exhibit the same pattern of genetic interactions; mutations in MDM20 are synthetically lethal with mutations in BEM2 and MYO2 but not SAC6. Although MDM20 and TPM1 are both required for the formation and/or stabilization of actin cables, mutations in these genes disrupt mitochondrial inheritance and nuclear segregation to different extents. Thus, Mdm20p and Tpm1p may act in vivo to establish molecular and functional heterogeneity of the actin cytoskeleton.

Show MeSH
Related in: MedlinePlus