The Preprotein Translocation Channel of the Outer Membrane of Mitochondria

Abstract

The preprotein translocase of the outer membrane of mitochondria (TOM complex) facilitates the recognition, insertion, and translocation of nuclear-encoded mitochondrial preproteins. We have purified the TOM complex from Neurospora crassa and analyzed its composition and functional properties. The TOM complex contains a cation-selective high-conductance channel. Upon reconstitution into liposomes, it mediates integration of proteins into and translocation across the lipid bilayer. TOM complex particles have a diameter of about 138 A, as revealed by electron microscopy and image analysis; they contain two or three centers of stain-filled openings, which we interpret as pores with an apparent diameter of about 20 A. We conclude that the structure reported here represents the protein-conducting channel of the mitochondrial outer membrane.

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“…Thus, the Tim23 channel is not simply a cylindrical pore; rather, its internal and external diameters differ considerably. At its narrowest point, the Tim23 channel is smaller than both the Tom40 channel 5,6,26 and the functional translocon of the endoplasmic reticulum (∼20-50 Å) [30][31][32] but is similar to the average diameter of the polypeptide exit channel of the ribosome large subunit (15 Å) 33 . The restriction zone diameter of the Tim23 channel is just large enough to accommodate one polypeptide chain in an α-helical conformation but not large enough to contain two α-helices at the same time.…”

“…The diameter of the Tom40 translocation pore has been experimentally estimated at ∼20-22 Å, indicating that a polypeptide in an α-helical conformation can easily be translocated through it and that two α-helices (for example, a preprotein in a loop formation) could even be translocated together 5,6,26 . The rela- In the presence of preimmune antibodies, the current-voltage relationship was superimposable with the control trace.…”

“…Two distinct protein complexes are essential for this process: the translocase of the outer membrane (TOM complex) and a translocase of the inner membrane specific for preproteins containing a mitochondrial presequence (TIM23 complex) [1][2][3][4] . The essential protein Tom40 forms a continuous hydrophilic channel in the outer membrane that allows preproteins to pass across the membrane 5,6 . Other known Tom components function as receptors for preprotein recognition or in the assembly and stability of the TOM complex.…”

Electro-osmotic flow through a nanopore

“…Living cells and intracellular organelles are usually bounded by lipid membranes containing nanopores constructed of membrane-bound proteins. The transport of small molecules and polymers across such nanopores is a very common feature in living cells and is essential to their normal function (Alberts et al 1994;Pfanner & Neupert 1990;Matouschek et al 2000;Martin et al 1991;Künkele et al 1998). Synthetic nanopores (Li et al 2003;Storm et al 2005a,b;Smeets et al 2006;Hall et al 2010;Garaj et al 2010;Schneider et al 2010) have been the focus of much interest in recent years following the demonstration of their use as effective single molecule sensors (Kasianowicz et al 1996).…”

Protein Import Into Mitochondria

“…The additional TOM core subunits Tom22, Tom7, Tom6, and Tom5 are necessary for the stability and the dynamic regulation of the complex (41,45,48). Recently, the structure of isolated TOM holo and core complexes from Neurospora crassa were elucidated by electron microscopy and tomographic 3D reconstitution (19,49). The analysis revealed that one TOM holo complex contains two or three pores, whereas the TOM core complex contains only two pores.…”