Endoplasmic Reticulum

ID #2438

I have a few questions: 1)So the SRP recognizes the signal sequence and brings it to the ER, importin recognizes an NLS to brign something to the nucleus. mitochondria have the pre sequence, but have we disuccsed how it gets to the nucleus? 2)What is the difference between the cytosolic/mitochondrial HSP70 despite their difference in location? 3)What is the point of Tom? couldnt the proteins just get into the mitochondria through the porins? 4)In one of our homework problems, could you clarify this statement: "Cleavage of a protein's mitochondrial presequence or transit sequence is done by the transit peptidase using ATP as an energy source." I know the transit peptidase clips the sequence, but I couldn't find anything in the book about the energy source. If it's not ATP is it GTP?

To answer your questions: 
1) I assume you are asking about how proteins with a presequence get into the mitochondria, not the nucleus. And yes, the movement of proteins through Tom and Tim was discussed in lecture 25. Proteins targeted to the mitochondria are not escorted through Tom and Tim the way importins bring proteins bearing NLS's into the nucleus, for example. They must pass through Tom and Tim unfolded and do not have protein "escorts." 
2)Cytosolic and mitochondrial Hsp70 are different chaperones. They are the same size (which is the Hsp70 designation) but are different chaperones and are found in different locations. 
3) Since mitochondria have a double lipid bilayer, two translocons are required to fully import proteins into the mitochondrial matrix. Porins are too small to allow proteins through. Porins allow passage of molecules smaller than 1000 Daltons (1 kDa). The average protein is 35 kDa, so even unfolded a protein would not be able to pass through a porin. 
4) No energy source is required for cleavage of the presequence by the transit peptidase.

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