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Mmm1p, a mitochondrial outer membrane protein, is connected to mitochondrial DNA (mtDNA) nucleoids and required for mtDNA stability.

Hobbs AE, Srinivasan M, McCaffery JM, Jensen RE - J. Cell Biol. (2001)

Bottom Line: We found that Mmm1p-GFP is located in small, punctate structures on the mitochondrial outer membrane, adjacent to a subset of matrix-localized mitochondrial DNA nucleoids.We also found that the temperature-sensitive mmm1-1 mutant was defective in transmission of mitochondrial DNA to daughter cells immediately after the shift to restrictive temperature.Moreover, we found that mitochondrial inner membrane structure is dramatically disorganized in mmm1 disruption strains.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell Biology and Anatomy, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA.

ABSTRACT
In the yeast Saccharomyces cerevisiae, mitochondria form a branched, tubular reticulum in the periphery of the cell. Mmm1p is required to maintain normal mitochondrial shape and in mmm1 mutants mitochondria form large, spherical organelles. To further explore Mmm1p function, we examined the localization of a Mmm1p-green fluorescent protein (GFP) fusion in living cells. We found that Mmm1p-GFP is located in small, punctate structures on the mitochondrial outer membrane, adjacent to a subset of matrix-localized mitochondrial DNA nucleoids. We also found that the temperature-sensitive mmm1-1 mutant was defective in transmission of mitochondrial DNA to daughter cells immediately after the shift to restrictive temperature. Normal mitochondrial nucleoid structure also collapsed at the nonpermissive temperature with similar kinetics. Moreover, we found that mitochondrial inner membrane structure is dramatically disorganized in mmm1 disruption strains. We propose that Mmm1p is part of a connection between the mitochondrial outer and inner membranes, anchoring mitochondrial DNA nucleoids in the matrix.

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Normal inner membrane cristae morphology is lost in mmm1 disruption mutants. Wild-type (RJ485), wild-type rho° (YHS92), and mmm1::URA3 (RJ483) cells were grown at 24°C to mid-log phase, fixed, and thin sections were examined by electron microscopy. (A) Wild-type cells; (B) wild-type rhoo cells; (C) mmm1::URA3 cells. m, mitochondria; n, nucleus. Bar: 0.1 μm.
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Figure 7: Normal inner membrane cristae morphology is lost in mmm1 disruption mutants. Wild-type (RJ485), wild-type rho° (YHS92), and mmm1::URA3 (RJ483) cells were grown at 24°C to mid-log phase, fixed, and thin sections were examined by electron microscopy. (A) Wild-type cells; (B) wild-type rhoo cells; (C) mmm1::URA3 cells. m, mitochondria; n, nucleus. Bar: 0.1 μm.

Mentions: To further investigate the function of Mmm1p, we examined the morphology of mitochondria in mmm1 mutants by electron microscopy. Instead of the temperature-sensitive mmm1-1 mutant, we used cells carrying a mutation in MMM1. mmm1::URA3 disruptions show altered mitochondrial shape and fail to grow on nonfermentable carbon sources at 24°, 30°, and 37°C (Burgess et al. 1994). Wild-type and mmm1::URA3 cells were grown on raffinose-containing medium at 24°C, fixed, stained, embedded, and sectioned. As shown in Fig. 7 A, wild-type cells contained tubular-shaped mitochondria seen cut in both longitudinal and cross section. Inner membrane cristae appeared as simple membrane invaginations or tubules. In contrast, mmm1::URA3 cells contained very large organelles whose inner membrane morphology was drastically altered. Instead of tubular-shaped cristae with simple attachments to the outer membrane, the inner membrane in mitochondria of mmm1::URA3 cells was collapsed, often forming stacks of membrane sheets inside the organelle (Fig. 7 C). In other mmm1::URA3 mitochondria, circular-shaped inner membrane structures with expanded intermembrane spaces were seen (Fig. 7 C). The aberrant inner membrane structures were often very long, sometimes stretching for over a micron in the mmm1::URA3 mitochondria. Thus, the lack of the outer membrane protein, Mmm1p, caused a dramatic alteration in the structure of the mitochondrial inner membrane. The disruption of inner membrane structure is not simply the result of mtDNA loss. Wild-type cells that lack mtDNA show elongated tubules similar to those in wild-type cells (Fig. 7 B). Although the number of cristae are reduced compared with wild-type, rho° mitochondria do not show the severe defects in inner membrane structure seen in mmm1::URA3 cells. The mitochondrial morphology seen in mmm1::URA3 cells differs from that seen in mmm1-1 cells shifted to the nonpermissive temperature (Burgess et al. 1994). In our previous studies, mmm1-1 cells were incubated for 3 h at 37°C. In these cells, mitochondria were seen as spherical organelles, but inner membrane structure was not dramatically altered. However, we find that the mitochondrial inner membrane in mmm1-1 cells shifted for longer times to the restrictive temperature show many of the dramatic changes seen in Fig. 7 (A. Aiken Hobbs and J.M. McCaffery, unpublished observations). Why in mmm1-1 cells the collapse in external mitochondria shape is seen within 90 min, but the loss of inner membrane structure takes many hours is unclear.


Mmm1p, a mitochondrial outer membrane protein, is connected to mitochondrial DNA (mtDNA) nucleoids and required for mtDNA stability.

Hobbs AE, Srinivasan M, McCaffery JM, Jensen RE - J. Cell Biol. (2001)

Normal inner membrane cristae morphology is lost in mmm1 disruption mutants. Wild-type (RJ485), wild-type rho° (YHS92), and mmm1::URA3 (RJ483) cells were grown at 24°C to mid-log phase, fixed, and thin sections were examined by electron microscopy. (A) Wild-type cells; (B) wild-type rhoo cells; (C) mmm1::URA3 cells. m, mitochondria; n, nucleus. Bar: 0.1 μm.
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Figure 7: Normal inner membrane cristae morphology is lost in mmm1 disruption mutants. Wild-type (RJ485), wild-type rho° (YHS92), and mmm1::URA3 (RJ483) cells were grown at 24°C to mid-log phase, fixed, and thin sections were examined by electron microscopy. (A) Wild-type cells; (B) wild-type rhoo cells; (C) mmm1::URA3 cells. m, mitochondria; n, nucleus. Bar: 0.1 μm.
Mentions: To further investigate the function of Mmm1p, we examined the morphology of mitochondria in mmm1 mutants by electron microscopy. Instead of the temperature-sensitive mmm1-1 mutant, we used cells carrying a mutation in MMM1. mmm1::URA3 disruptions show altered mitochondrial shape and fail to grow on nonfermentable carbon sources at 24°, 30°, and 37°C (Burgess et al. 1994). Wild-type and mmm1::URA3 cells were grown on raffinose-containing medium at 24°C, fixed, stained, embedded, and sectioned. As shown in Fig. 7 A, wild-type cells contained tubular-shaped mitochondria seen cut in both longitudinal and cross section. Inner membrane cristae appeared as simple membrane invaginations or tubules. In contrast, mmm1::URA3 cells contained very large organelles whose inner membrane morphology was drastically altered. Instead of tubular-shaped cristae with simple attachments to the outer membrane, the inner membrane in mitochondria of mmm1::URA3 cells was collapsed, often forming stacks of membrane sheets inside the organelle (Fig. 7 C). In other mmm1::URA3 mitochondria, circular-shaped inner membrane structures with expanded intermembrane spaces were seen (Fig. 7 C). The aberrant inner membrane structures were often very long, sometimes stretching for over a micron in the mmm1::URA3 mitochondria. Thus, the lack of the outer membrane protein, Mmm1p, caused a dramatic alteration in the structure of the mitochondrial inner membrane. The disruption of inner membrane structure is not simply the result of mtDNA loss. Wild-type cells that lack mtDNA show elongated tubules similar to those in wild-type cells (Fig. 7 B). Although the number of cristae are reduced compared with wild-type, rho° mitochondria do not show the severe defects in inner membrane structure seen in mmm1::URA3 cells. The mitochondrial morphology seen in mmm1::URA3 cells differs from that seen in mmm1-1 cells shifted to the nonpermissive temperature (Burgess et al. 1994). In our previous studies, mmm1-1 cells were incubated for 3 h at 37°C. In these cells, mitochondria were seen as spherical organelles, but inner membrane structure was not dramatically altered. However, we find that the mitochondrial inner membrane in mmm1-1 cells shifted for longer times to the restrictive temperature show many of the dramatic changes seen in Fig. 7 (A. Aiken Hobbs and J.M. McCaffery, unpublished observations). Why in mmm1-1 cells the collapse in external mitochondria shape is seen within 90 min, but the loss of inner membrane structure takes many hours is unclear.

Bottom Line: We found that Mmm1p-GFP is located in small, punctate structures on the mitochondrial outer membrane, adjacent to a subset of matrix-localized mitochondrial DNA nucleoids.We also found that the temperature-sensitive mmm1-1 mutant was defective in transmission of mitochondrial DNA to daughter cells immediately after the shift to restrictive temperature.Moreover, we found that mitochondrial inner membrane structure is dramatically disorganized in mmm1 disruption strains.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell Biology and Anatomy, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA.

ABSTRACT
In the yeast Saccharomyces cerevisiae, mitochondria form a branched, tubular reticulum in the periphery of the cell. Mmm1p is required to maintain normal mitochondrial shape and in mmm1 mutants mitochondria form large, spherical organelles. To further explore Mmm1p function, we examined the localization of a Mmm1p-green fluorescent protein (GFP) fusion in living cells. We found that Mmm1p-GFP is located in small, punctate structures on the mitochondrial outer membrane, adjacent to a subset of matrix-localized mitochondrial DNA nucleoids. We also found that the temperature-sensitive mmm1-1 mutant was defective in transmission of mitochondrial DNA to daughter cells immediately after the shift to restrictive temperature. Normal mitochondrial nucleoid structure also collapsed at the nonpermissive temperature with similar kinetics. Moreover, we found that mitochondrial inner membrane structure is dramatically disorganized in mmm1 disruption strains. We propose that Mmm1p is part of a connection between the mitochondrial outer and inner membranes, anchoring mitochondrial DNA nucleoids in the matrix.

Show MeSH
Related in: MedlinePlus