Question

Essay #1a: Describe the pathway that newly synthesized polypeptides take. Be sure to also include what cytoskeleton protein is involved, and explain whether the various steps is active or passive.

Answer 1

Newly made proteins travel from the ribosome in the ER to the insides of the ER itself if they have an special peptide sequence that indicates it has to go inside ER. This translocation is an active process as it needs the use of a ATPase. Sometimes they get automatically folded for their function but some other times they need chaperones to correctly fold.

Proteins then need to be translocated by either being inside a a lipid vesicle or incrusted inside the membrane of such vesicles. This vesicles get formed by the construction of a protein coat in the membrane surface, such coat changes the membrane configuration and it starts to form and separte a vesicle. This process is also active, needs energy to be made. After this, the vesicle need to get correctly transported, for this we are going to use the microtubules in the cytoskeleton.

Cytoskeleton is all across the cell, so there are many elements that go from one place in the cell to another. Vesicles move by being taken by a kinesin. Kinesin is a protein that has an elongated structure and two "feet", the elongated structure attaches to the vesicle and starts to "walk" all along a microtubule until they reach the final destination. This is an active process, as each kinesin "step" needs energy to be made.

Many proteins reach Golgi and undergo some more modifications, some others stay in the ER, others go outside the cell after being modificated by Golgi. Bit have in mind that any other trasnport event will use the same Kinesin/microtubule mechanism.