Protein Translocation How Hsp70 Pulls It Off

In an alternative mode, the Sec61 protein associates with a tetrameric Sec62/63p complex to form the SEC complex, a channel active in posttranslational transloca-heat shock 70 protein (Hsp70) called BiP, or Kar2p in yeast (Lyman and Schekman, 1997). BiP and the SEC complex are the only components required for translo-cation in a reconstituted system (Panzner et al., 1995); of A significant fraction of the proteins synthesized in a these components only BiP consumes metabolic energy cell are inserted into or transported across a membrane by means of ATP hydrolysis that is thought to provide through complex and energy-coupled translocation ma-directionality to the translocation process (Matlack et chines. Each translocation machine fulfils a variety of al., 1999). tasks. First, it must select precursors that carry the cor-The mitochondrial envelope consists of two mem-rect targeting signal. Then, it must provide a pathway branes. Receptors in the outer membrane serve to guide for those precursors to cross the membrane without the presequence of a mitochondrial precursor into an compromising the membrane permeability barrier. Next outer membrane pore (Kü nkele et al., 1998). A separate the precursor must be actively transported from the channel-forming unit exists in the inner membrane cytoplasmic side to the trans side of the membrane. (Bö mer et al., 1997). The electrochemical potential Finally the precursor must be properly processed and, if across the inner membrane is required for activation of necessary, must be assisted in folding. The best-studied this pore and insertion of the presequence. Full translo-protein translocation systems in eukaryotes are those cation requires the activity of a Hsp70 protein in the localized in the endoplasmic reticulum (ER) membrane matrix called mtHsp70 (Voisine et al., 1999). and in the envelope of mitochondria. A central and still Fusion proteins have been employed in the mitochon-incompletely resolved question with respect to these drial import system to investigate the requirement for translocation systems is how metabolic energy is used protein unfolding during translocation. These fusions to drive the movement of proteins. Two models, biased consist of a targeting sequence and a passenger protein, diffusion and an active pulling process, have been pro-such as dihydrofolate reductase (DHFR), representing posed. Though the mechanisms are not mutually exclu-a folded domain whose stability can be manipulated. A sive, a certain amount of heat has been generated in fusion protein of a mitochondrial presequence with the process of developing experimental distinctions be-DHFR was imported by mitochondria, …