Moreover, mitochondria have no exact analog of the SRP system that results in a direct physical connection between the ribosome and the translocon of the outer mitochondrial membrane. However, in organisms as diverse as budding yeast (S accharomyces cerevisiae) and humans, a substantial number of proteins are translocated post-translationally into the ER. This precise docking provides a direct conduit for the nascent polypeptide chain from the ribosome exit tunnel through the channel in the membrane-imbedded translocon. Via action of signal recognition particle (SRP) binding to targeting sequences at the N-terminus of an ER-destined protein, the translating ribosome docks directly onto the translocon of the ER membrane. Coupling of protein translation and protein translocation minimizes the issue of tertiary structure hindering passage through the translocation channel, while using the “force” of protein synthesis to drive directional movement across the membrane. In addition, protein movement must not only be vectorial, that is, unidirectional from the cytosol into the organelle, it must also be efficient to keep up with the heavy cellular demand for organelle function.įor many ER proteins, the co-translational nature of the translocation process overcomes such hurdles. Thus, postponing folding, yet preventing aggregation, of a protein is necessary for its efficient translocation. They are able to accommodate only a completely unfolded chain or, at most, an α-helix. The protein complexes embedded in the membrane, referred to as translocases or translocons, through which the proteins must pass, have narrow channels. Translocation of proteins into the endoplasmic reticulum (ER) and mitochondria is especially demanding. Proteins synthesized on cytosolic ribosomes and translocated across membranes into organelles play critical roles in cell and organismal physiology.
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