Mitochondrial Substructure(2)The cartoons in theprevious section showed cytochrome C lying just outside the inner
membrane.Note
that the enzyme reaction product is confined to the cristae and in factdelineates the cristae. This process is diagrammed in the cartoon. Thisinvolves a furrowing of the inner and then the outer
membrane as if someone waspinching the mitochondrion. The electron micrograph in the following figureshows such a center. 126: 1361-1373, 1994. They showed the result of the removalof an outer membrane protein from mitochondria called MDM10. This figure showsthe results. The DNA iscircular and lies in the matrix.in punctate structures called"nucleoids". Most of the lipid isimported (recall the lectures on lipid addition to membranes). The highlightedlabels are drugs that can be used to block the process and test the source ofthe mitochondrial protein.The outer membrane of themitochondria contains the protein "porin". Indeed, the smallmolecules actually equilibrate between the outer membrane and the cytosol. Themachinery in the outer membrane is called the Tom
complex (Translocator outermembrane) and that for the inner membrane is called the Tim complex (TranslocatorInner Membrane). In the above cartoon, thereceptor is represented as a blue oval in which the signal peptide is inserted.The receptors then bring the protein to the region containing the translocatorproteins. It is called the General Import Pore (GIP) and it facilitates the translocationof the presequence of the protein across the outer membrane. (the GIP is madeof Tom40, Tom5, Tom 6, and Tom7). Tom40 appears to be the core element of thepore and forms oligomers. They then enter the matrix using the porecomplex made of Tim23 and Tim17 which are in the inner membrane. Theprotein then enters the matrix where the cleavable preprotein is clipped off bya protease, MPP. mt-hsp70 in the matrix works with Tim44 to complete thefull transfer to the matrix. These are like the Type I membrane proteinsdescribed in the unit on the rough endoplasmic reticulum.Then, after they transitthe outer membrane via the GIP complex, they enter the special Timpathway. Tim23 is one of the inner membranetranslocator proteins. The signal for targeting tomitochondria is found in at least two of the hydrophobic transmembranesegments.Molecular Biology of theCell 10: 2461-2474 (1999). It required chaperones and ATP toget to the matrix.However,the distribution of the charged amino acids helps orient the protein so thatthe positive charges are in the matrix. This is how the cytochromes in therespiratory chain or the elementary particlesare inserted bymitochondrial actions. Other Tom proteins involved,but Tom40 is the core of the translocator channel.This attracts theprotein (the signal is positively charged). Tim 44 and hsp70 in the matrixcontinue to guide and pull the protein through the pore. Step 6. Presequence iscleaved in the matrix.series,where the number designates the protein number. An important protein inthe recognition of the signal peptide and its binding to the receptor is called"MOM19". Then MOM22 helps the protein to pass from the receptorbinding site to the insertion point at the outer membrane. So, the longer the cellsgrow in the drug, the more drug-resistant mutant cells will be found. They showthe result of the absent MOM19 protein. Mitochondriacontain two membranes, separated by a space. Both are the typical"unit membrane" (railroad track) in structure. Inside the spaceenclosed by the inner membrane is the matrix. This appears moderatelydense and one may find strands of DNA, ribosomes, or small granules in thematrix. The mitochondria are able to code for part of their proteins with thesemolecular tools. The above cartoon shows the diagram of the mitochondrialmembranes and the enclosed compartment.