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A first glimpse at the structure of the TOM translocase from the mitochondrial outer membrane.

Verschoor A, Lithgow T - J. Cell Biol. (1999)

View Article: PubMed Central - PubMed

Affiliation: Wadsworth Center, New York State Department of Health, Albany, New York 12201, USA.

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The TOM complex in the mitochondrial outer membrane consists of at least six distinct protein subunits, and functions as the gateway through which nuclear-encoded polypeptides can be imported into a mitochondrion... In the tomographic type of reconstruction that was used, resolution is anisotropic, and the geometry is such that the region of lowest resolution corresponds to only those regions of the complex that might protrude from the bilayer, towards either the cytosol or the intermembrane space... Nonetheless, this first glimpse at the TOM complex in three dimensions provides a foundation for further studies aimed towards a full understanding of this fascinating molecular machine... Based on the 3D structure of the TPR domains of protein phosphatase 5, Das et al. 1998 demonstrated that eight TPR units would form a tall helix surrounding a central channel... Another possibility is that the presence of the Tom20 and Tom70 subunits in the holo complex induces a docking or rearrangement of the Tom40 subunits to form three channels in the plane of what would be the mitochondrial membrane... Beyond this study of a mitochondrial outer membrane complex, EM has been applied to structural analysis of a host of biologically important complexes... Indeed, the next logical step for the solubilized TOM complex will be cryoimaging and 3D reconstruction by single-particle methods... Spahn, R.A... Grassucci, J... Frank, and P... Penczek, manuscript in preparation) or 5.0–7.8 Å resolution crystallographic maps of the intact ribosome or ribosomal subunits (Cate et al. 1999; Clemons et al. 1999; Ban et al. 1999; Fig. 1 B)... Now that several laboratories are progressing towards atomic resolution domain structures for components of the TOM complex, the same strategy could well provide a means to visualize the intact protein translocation machinery in three dimensions, with the phospholipid bilayer stripped away.

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A, Alignment of the nascent polypeptide tunnel and translocation pore of the Sec61 complex. Cryo-EM was used to visualize the interaction of a solubilized membrane protein, intractable to crystallization, with the ribosome. The complexes were reconstituted in vitro using purified yeast ribosomes and Sec61 complex solubilized from ER with detergent. The path followed by the nascent polypeptide as it exits the ribosome and crosses the ER can be traced in the transverse section (dashed line). The 40S and 60S subunits of the ribosome are labeled. Modified after Beckmann et al. 1997. B, Atomic resolution structures for components of a macromolecular complex, such as the ribosome, can be fitted into a lower resolution map of the entire complex (derived from either X-ray or cryo-EM), and their detailed interactions can be interpreted. The ribosomal proteins S4, S5, and S8 have been crystallized, and their structures resolved at atomic resolution. The crystal structures are shown fitted to the 5.5 Å X-ray map of the 30S ribosomal subunit from Thermus thermophilus. B, Ribosomal body landmark; S, ribosomal shoulder landmark; H, ribosomal head landmark; and P, ribosomal platform landmark. Modified after Clemons et al. 1999.
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Figure 1: A, Alignment of the nascent polypeptide tunnel and translocation pore of the Sec61 complex. Cryo-EM was used to visualize the interaction of a solubilized membrane protein, intractable to crystallization, with the ribosome. The complexes were reconstituted in vitro using purified yeast ribosomes and Sec61 complex solubilized from ER with detergent. The path followed by the nascent polypeptide as it exits the ribosome and crosses the ER can be traced in the transverse section (dashed line). The 40S and 60S subunits of the ribosome are labeled. Modified after Beckmann et al. 1997. B, Atomic resolution structures for components of a macromolecular complex, such as the ribosome, can be fitted into a lower resolution map of the entire complex (derived from either X-ray or cryo-EM), and their detailed interactions can be interpreted. The ribosomal proteins S4, S5, and S8 have been crystallized, and their structures resolved at atomic resolution. The crystal structures are shown fitted to the 5.5 Å X-ray map of the 30S ribosomal subunit from Thermus thermophilus. B, Ribosomal body landmark; S, ribosomal shoulder landmark; H, ribosomal head landmark; and P, ribosomal platform landmark. Modified after Clemons et al. 1999.

Mentions: In addition to revealing the structure of the eukaryotic ribosome, images have been reconstructed that depict how the ribosome docks with the Sec61 complex (Beckmann et al. 1997; Fig. 1 A). The 3D structure of the complex provides a framework in which to understand the multiple and sequential interactions made by a nascent polypeptide as it leaves a cytoplasmic ribosome to be translocated through an intracellular membrane. Like the structure of the TOM complex, the ribosome–Sec61 structure prompts new experiments and new ideas on the function and mechanics of the system.


A first glimpse at the structure of the TOM translocase from the mitochondrial outer membrane.

Verschoor A, Lithgow T - J. Cell Biol. (1999)

A, Alignment of the nascent polypeptide tunnel and translocation pore of the Sec61 complex. Cryo-EM was used to visualize the interaction of a solubilized membrane protein, intractable to crystallization, with the ribosome. The complexes were reconstituted in vitro using purified yeast ribosomes and Sec61 complex solubilized from ER with detergent. The path followed by the nascent polypeptide as it exits the ribosome and crosses the ER can be traced in the transverse section (dashed line). The 40S and 60S subunits of the ribosome are labeled. Modified after Beckmann et al. 1997. B, Atomic resolution structures for components of a macromolecular complex, such as the ribosome, can be fitted into a lower resolution map of the entire complex (derived from either X-ray or cryo-EM), and their detailed interactions can be interpreted. The ribosomal proteins S4, S5, and S8 have been crystallized, and their structures resolved at atomic resolution. The crystal structures are shown fitted to the 5.5 Å X-ray map of the 30S ribosomal subunit from Thermus thermophilus. B, Ribosomal body landmark; S, ribosomal shoulder landmark; H, ribosomal head landmark; and P, ribosomal platform landmark. Modified after Clemons et al. 1999.
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Related In: Results  -  Collection

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Figure 1: A, Alignment of the nascent polypeptide tunnel and translocation pore of the Sec61 complex. Cryo-EM was used to visualize the interaction of a solubilized membrane protein, intractable to crystallization, with the ribosome. The complexes were reconstituted in vitro using purified yeast ribosomes and Sec61 complex solubilized from ER with detergent. The path followed by the nascent polypeptide as it exits the ribosome and crosses the ER can be traced in the transverse section (dashed line). The 40S and 60S subunits of the ribosome are labeled. Modified after Beckmann et al. 1997. B, Atomic resolution structures for components of a macromolecular complex, such as the ribosome, can be fitted into a lower resolution map of the entire complex (derived from either X-ray or cryo-EM), and their detailed interactions can be interpreted. The ribosomal proteins S4, S5, and S8 have been crystallized, and their structures resolved at atomic resolution. The crystal structures are shown fitted to the 5.5 Å X-ray map of the 30S ribosomal subunit from Thermus thermophilus. B, Ribosomal body landmark; S, ribosomal shoulder landmark; H, ribosomal head landmark; and P, ribosomal platform landmark. Modified after Clemons et al. 1999.
Mentions: In addition to revealing the structure of the eukaryotic ribosome, images have been reconstructed that depict how the ribosome docks with the Sec61 complex (Beckmann et al. 1997; Fig. 1 A). The 3D structure of the complex provides a framework in which to understand the multiple and sequential interactions made by a nascent polypeptide as it leaves a cytoplasmic ribosome to be translocated through an intracellular membrane. Like the structure of the TOM complex, the ribosome–Sec61 structure prompts new experiments and new ideas on the function and mechanics of the system.

View Article: PubMed Central - PubMed

Affiliation: Wadsworth Center, New York State Department of Health, Albany, New York 12201, USA.

AUTOMATICALLY GENERATED EXCERPT
Please rate it.

The TOM complex in the mitochondrial outer membrane consists of at least six distinct protein subunits, and functions as the gateway through which nuclear-encoded polypeptides can be imported into a mitochondrion... In the tomographic type of reconstruction that was used, resolution is anisotropic, and the geometry is such that the region of lowest resolution corresponds to only those regions of the complex that might protrude from the bilayer, towards either the cytosol or the intermembrane space... Nonetheless, this first glimpse at the TOM complex in three dimensions provides a foundation for further studies aimed towards a full understanding of this fascinating molecular machine... Based on the 3D structure of the TPR domains of protein phosphatase 5, Das et al. 1998 demonstrated that eight TPR units would form a tall helix surrounding a central channel... Another possibility is that the presence of the Tom20 and Tom70 subunits in the holo complex induces a docking or rearrangement of the Tom40 subunits to form three channels in the plane of what would be the mitochondrial membrane... Beyond this study of a mitochondrial outer membrane complex, EM has been applied to structural analysis of a host of biologically important complexes... Indeed, the next logical step for the solubilized TOM complex will be cryoimaging and 3D reconstruction by single-particle methods... Spahn, R.A... Grassucci, J... Frank, and P... Penczek, manuscript in preparation) or 5.0–7.8 Å resolution crystallographic maps of the intact ribosome or ribosomal subunits (Cate et al. 1999; Clemons et al. 1999; Ban et al. 1999; Fig. 1 B)... Now that several laboratories are progressing towards atomic resolution domain structures for components of the TOM complex, the same strategy could well provide a means to visualize the intact protein translocation machinery in three dimensions, with the phospholipid bilayer stripped away.

Show MeSH