You have isolated a protein from a plant, which localizes to mitochondria after endocytosis by mammalian cells. The polypeptide aggregates and forms large channels in the mitochondrial outer membrane, releasing proteins from the inter-membranous space into the cytoplasm. a) How will treatment with this polypeptide affect mammalian cells in culture? b) If this polypeptide is injected into a cell which is double defective for Bax and Bak, will the cell undergo apoptosis?
Answers
Answer:
Explanation:
There are two subcompartments in mitochondria: the internal matrix space and the intermembrane space. These compartments are formed by the two concentric mitochondrial membranes: the inner membrane, which forms extensive invaginations, the cristae, and encloses the matrix space, and the outer membrane, which is in contact with the cytosol (Figure 12-22A). Chloroplasts have the same two subcompartments plus an additional subcompartment, the thylakoid space, which is surrounded by the thylakoid membrane (Figure 12-22B). Each of the subcompartments in mitochondria and chloroplasts contains a distinct set of proteins.
The subcompartments of mitochondria and chloroplasts. In contrast to the cristae of mitochondria (A), the thylakoids of chloroplasts (B) are not connected to the inner membrane and therefore form a compartment with a separate internal space (see Figure (more...)
New mitochondria and chloroplasts are produced by the growth of preexisting organelles followed by fission (discussed in Chapter 14). Their growth depends mainly on the import of proteins from the cytosol. This requires that proteins be translocated across a number of membranes in succession and end up in the appropriate place. How this occurs is the subject of this section.
Translocation into the Mitochondrial Matrix Depends on a Signal Sequence and Protein Translocators
Proteins imported into the matrix of mitochondria are usually taken up from the cytosol within seconds or minutes of their release from ribosomes. Thus, in contrast to the protein translocation into the ER described later, mitochondrial proteins are first fully synthesized as precursor proteins in the cytosol and then translocated into mitochondria by a posttranslational mechanism. Most of the mitochondrial precursor proteins have a signal sequence at their N terminus that is rapidly removed after import by a protease (the signal peptidase) in the mitochondrial matrix. The signal sequences are both necessary and sufficient for import of the proteins that contain them: through the use of genetic engineering techniques, these signals can be linked to any cytosolic protein to direct the protein into the mitochondrial matrix. Sequence comparisons and physical studies of different matrix signal sequences suggest that their common feature is the propensity to fold into an amphipathic α helix, in which positively charged residues are clustered on one side of the helix, while uncharged hydrophobic residues are clustered on the opposite side (Figure 12-23). This configuration—rather than a precise amino acid sequence—is recognized by specific receptor proteins that initiate protein translocation.
A signal sequence for mitochondrial protein import. Cytochrome oxidase is a large multiprotein complex located in the inner mitochondrial membrane, where it functions as the terminal enzyme in the electron-transport chain (discussed in Chapter 14). (A) (more...)
Protein translocation across mitochondrial membranes is mediated by multi-subunit protein complexes that function as protein translocators: the TOM complex functions across the outer membrane, and two TIM complexes, the TIM23 and TIM22 complexes, function across the inner membrane (Figure 12-24). TOM and TIM stand for translocase of the outer and inner mitochondrial membranes, respectively. These complexes contain some components that act as receptors for mitochondrial precursor proteins and other components that form the translocation channel. The TOM complex is required for the import of all nucleus-encoded mitochondrial proteins. It initially transports their signal sequences into the intermembrane space and helps to insert transmembrane proteins into the outer membrane. The TIM23 complex then transports some of these proteins into the matrix space, while helping to insert transmembrane proteins into the inner membrane. The TIM22 complex mediates the insertion of a subclass of inner membrane proteins, including the carrier protein that transports ADP, ATP, and phosphate. A third protein translocator in the inner mitochondrial membrane, the OXA complex, mediates the insertion of inner membrane proteins that are synthesized within the mitochondria. It also helps to insert some proteins that are initially transported into the matrix by the TOM and TIM complexes.