Pathways for the incorporation of polypeptides into mitochondria. A Pathways for the incorporation of polypeptides into mitochondria. A. Incorporation of matrix proteins. A targeting signal located within a cleavable presequence at the N-terminus of the polypeptide (1) leads first to the association of the polypeptide first with the outer membrane (2) and then, in the presence of a transmembrane potential (3), to the passage of the polypeptide into the matrix through a point of contact between the two membranes. In an intermediate stage (4), parts of the polypeptide have reached the matrix while others are still exposed on the mitochondrial surface. The presequence is removed by a matrix metalloprotease and (5) the final product may remain in the matrix (as is the case with ornithine transcarbamylase; OTC) or become peripherally associated with the inner membrane (as is the case with the β subunit of the F1 ATPase). B. Incorporation of a protein into the outer membrane. The polypeptide contains an N-terminal targeting signal that is immediately followed by a localization signal (1). As in A, the targeting signal leads to association of the polypeptide with the outer membrane (2), but the localization signal prevents passage through the inner membrane at the contact point (3 and 4). The localization signal is a hydrophobic segment similar to the halt-transfer signal of type I membrane proteins. In the figure, the targeting signal of the mature protein is depicted as exposed in the intermembrane space, but this has not been shown and the part of the protein that forms an amphipathic helix may also be embedded in the outer membrane. The model is based on experimental data obtained for a 70-kDa outer membrane protein (OMP). C and D. Two alternative pathways for the incorporation of a protein (such as cytochrome b 2) into the intermembrane space. C. In this case, the targeting signal leads to complete translocation of the polypeptide into the matrix, where it is cleaved by the metalloprotease (1 to 4). This exposes a hydrophobic localization signal which serves as an insertion signal to mediate translocation of the polypeptide across the inner membrane from the matrix to the intermembrane space (4 to 6). The protein is discharged into the intermembrane space as a result of cleavage of the localization-insertion signal by a second protease, which is located on the outer surface of the inner membrane (7). D. In this model, the localization signal serves to halt transfer of the polypeptide across the inner membrane (4), which is initiated by the targeting signal (1 to 3). The polypeptide is discharged into the intermembrane space after cleavage by the second protease (5), as in C. E and F. Alternative pathways for the incorporation into mitochondria of a protein, such as cytochrome c 1, which is associated with the inner membrane but is largely exposed in the intermembrane space. E. The mechanism operates as in C, but translocation from the matrix into the intermembrane space is halted (7) before completion by a segment located near the C-terminus of the protein. F. As in D, the localization signal halts translocation into the matrix (4). Before cleavage by the second protease (6), however, the C-terminal region of the polypeptide inserts into the inner membrane (5). (Based in part on ref.873 .) Source: The Biogenesis of Membranes and Organelles, The Online Metabolic and Molecular Bases of Inherited Disease Citation: Valle D, Beaudet AL, Vogelstein B, Kinzler KW, Antonarakis SE, Ballabio A, Gibson K, Mitchell G. The Online Metabolic and Molecular Bases of Inherited Disease; 2014 Available at: http://ommbid.mhmedical.com/DownloadImage.aspx?image=/data/Books/971/ch16fg38.png&sec=62648352&BookID=971&ChapterSecID=62647964&imagename= Accessed: October 19, 2017 Copyright © 2017 McGraw-Hill Education. All rights reserved