Preproteins, translocation channels

Kunkele KP et al. (1998) The preprotein translocation channel of the outer membrane of mitochondria. Cell 93 1009-1019... 

For most proteins of the ER, translocation is coupled with protein synthesis. The synthesis of the growing polypeptide from the ribosome docked onto the ER is sufficient to drive the forward movement of the preprotein into the organelle (Fig. 1 Walter andjohnson, 1994 Matlack et al., 1998). However, it was found that some ER-targeted proteins, especially those found in yeast, could be imported into this organelle in a posttranslational fashion. Thus, in such cases the ribosome can play no part in driving preprotein translocation. Indeed an in vitro ER protein import assay has been developed that utilizes fully synthesized preproteins (Hansen et al., 1986 Rothblatt and Meyer, 1986 Waters and Blobel, 1986), and at least one ER-directed preprotein has been shown to be folded in the cytosol before its translocation into the organelle (Paunola et al., 1998). It seems that for yeast, posttranslational import utilizes the same ER Sec61 translocation channel as that employed in the cotranslational system however, additional factors are also recruited to the complex (Fig. 1 Panzer etal., 1995). 

B. mtHsp70 and BiP are Concentrated at the Preprotein Exit Sites of Translocation Channels... 

In the Brownian ratchet model (Simon et al., 1992), Hsp70 molecules act to support unidirectional translocation of preproteins in a somewhat passive way. Brownian motion describes the random thermal motion of a system—in this case, the preprotein. If a preprotein is in transit at the translocation channel, Brownian motion will cause it to oscillate in an unbiased way. However, the binding of Hsp70 to the incoming preprotein at the exit site of the translocation channel prevents its backsliding. Hsp70 bound to preprotein is then released from its anchor (e.g., Sec63p... 

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