The General Import Pore

Within the completed genome sequences of I vaginalis, G. intestinalis, 

histolytica, Cryptosporidium and E. cuniculi, putative Tom40 homologues were only found in Cryptosporidium (Abrahamsen et al. 2004 Xu et al. 2004) and in E. cuniculi, which also has a putative Tom70 homologue (Katinka et al. 

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Dekker PJ, Ryan MT, Brix J, Muller H, Honlinger A, Pfanner N (1998) Preprotein translo-case of the outer mitochondrial membrane molecular dissection and assembly of the general import pore complex. Mol Cell Biol 18 6515-6524 Delgadillo MG, Liston DR, Niazi K, Johnson PJ (1997) Transient and selectable transformation of the parasitic protist Trichomonas vaginalis. Proc Natl Acad Sci USA 94 4716-4720... 

The majority of mitochondria-destined preproteins seem to be imported after completion of polypeptide synthesis (Pfanner et al., 1997 Herrmann and Neupert, 2000). Preproteins are targeted to the outer membrane, often via N-terminal signals, where they bind to translo-case components of the outer membrane (TOM). Preproteins subsequently translocate across the general import pore (Ryan and Pfanner, 1998). Extra complexity is introduced into matrix-imported preproteins because they must also translocate across the inner membrane (Fig. 2). This is achieved via interactions of the preproteins with the translocase... 

The Import receptors subsequently transfer the precursor proteins to an Import channel in the outer membrane. This channel, composed mainly of the Tom40 protein, is known as the general import pore because all known mitochondrial precursor proteins gain access to the interior compartments of the mitochondrion through this channel. When... 

Cytochrome c heme lyase, the enzyme responsible for the covalent attachment of heme to cytochrome c, illustrates a second pathway for targeting to the intermembrane space. In this pathway, the Imported protein is delivered directly to the Intermembrane space via the general import pore without Involvement of any Inner-membrane translocation factors... 

Proteins destined to the mitochondrial matrix bind to receptors on the outer mitochondrial membrane, and then are transferred to the general import pore (Tom40) in the outer membrane. Translocation occurs concurrently through the outer and inner membranes, driven by the proton-motive force across the inner membrane and ATP hydrolysis by the Hsc70 ATPase in the matrix (see Figure 16-26). 

Dietmeier, K., Honlinger, A., Bomer, U, Dekker, P.J., Eckerskorn, C., Lottspeich, R, Kubrich, M. Pfanner, N. (1997) Nature, 388, 195-200. Tom5 functionally links mitochondrial preprotein receptors to the general import pore. 

Since roughly 1000 stuck translocation intermediates can be observed in a typical yeast mitochondrion, it is thought that mitochondria have approximately 1000 general import pores for the uptake of mitochondrial proteins. 

The first thing to notice about these results is that the influence of the micropores reduces the effective diffusion coefficient below the value of the bulk diffusion coefficient for the macropore system. This is also clear in general from the forms of equations (10.44) and (10.48). As increases from zero, corresponding to the introduction of micropores, the variance of the response pulse Increases, and this corresponds to a reduction in the effective diffusion coefficient. The second important point is that the influence of the micropores on the results is quite small-Indeed it seems unlikely that measurements of this type will be able to realize their promise to provide information about diffusion in dead-end pores. 

The most important application of semi-permeable membranes is in separations based on reverse osmosis. These membranes generally have pores smaller than 1 nm. The pressure across the semi-permeable membranes for reverse osmosis is generally much larger than those for ultrafiltration, for example. This is because reverse osmosis is usually used for small molecules which have a much higher osmotic pressure, because of the higher number density, than the colloids separated in ultrafiltration. As a result reverse osmosis membranes have to be much more robust than ultrafiltration membranes. Since the focus of our discussion in this chapter will be on reverse osmosis based separations, we will describe these membranes in greater detail. 

Generally, optimizing the selectivity by choosing a gel medium of suitable pore size and pore size distribution is the single most important parameter. Examples of the effect of pore size on the separation of a protein mixture are given in Fig. 2.15. The gain in selectivity may then be traded for speed and/ or sample load. However, if the selectivity is limited, other parameters such as eluent velocity, column length, and sample load need to be optimized to yield the separation required. 

The model of clusters or ensembles of sites and bonds (secondary supramolecular structure), whose size and structure are determined on the scale of a process under consideration. At this level, the local values of coordination numbers of the lattices of pores and particles, that is, number of bonds per one site, morphology of clusters, etc. are important. Examples of the problems at this level are capillary condensation or, in a general case, distribution of the condensed phase, entered into the porous space with limited filling of the pore volume, intermediate stages of sintering, drying, etc. 

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