Protein Targeting

Homework 5

1) Compare and contrast the protein targeting to cytosol mitochondria and nuclear envelope.

a. Cytosolic proteins are all soluable and are translated by free ribosomes. There relatively free to difuse in the cell but can be localized prior to translation by transport proteins recongnizing the zip code of the 3' UTR of the mRNA.

b. Mitochondrial proteins are similar because they to are translated by the free ribosomes both soluble and membrain acociated proteins. The primary structures of these proteins are maintained in there unfolded state and transported to the mitochondria by cytosolic chaperons. The presquence determines where in the mitochondria the protein will go, outer membrane, intermembrane space, inner membrane, or matrix. The translocation of the primary polypeptide into the mitochondria is facilitated by the TOM and TIM for the outer and inner membrane translocation respectively.

c. The Nuclear envelope has pores that are largeenough to accommodate the transport of fully folded proteins. This is accomplished by the nuclear localizing signal located near the C terminus of the polypeptide chain. The NLS is recognized by importin б and в and then the cytoplasm filaments facilitate its entrance into the nuclear envelope.

2) Targeting proteins for the ER and the Lysosomes accomplished by membrane associated receptors. In the ER the proteins bind with their specific receptors and these receptors are gathered into one area in the membrane where COP II proteins gather and begin to cause a piece of the ER membrane to bud off. COP II proteins are associated with Sar1 GTP. The hydrolysis of the GTP disrupts the COP II coat after the vesicle has completely formed and the vesicle is transported to the cic Golgi network where it is sorted. Proteins the were erroneously taken form the ER as well as those bound progress through the cisterae