Question

After import into the ER, a transmembrane protein with no additional signals or tags will follow the:

lysosomal pathway

regulated secretory pathway

default secretory pathway

endocytic pathway

Answer 1

The correct answer is 3) default secretory pathway without any tags.

The transmembrane proteins after importing in endoplasmic reticulum proceeds to various pathways based upon their tags. The tags are the amino acids chain etc. which attached to the transmembrane protein acts as a signal or address to destination which will help in knowing which pathway to follow.

The secretory pathway also known as regulated pathway in which there is regulated secretion of vesicles near Pm (plasma membrane). The tags were present in the regulated secretory pathway. Lysosomal and endocytic pathway also involve tags which helps in locating the address of the transmembrane protein. Lysosomal pathway has sugar chain as the tag which will help the protein to signal and reaching its destination. Lysosomal pathway works along with the endocytic pathway the material first enters the early endosome stage of lysosome and after there is nothing important left it will go to secondary stage and eliminated using lysosomal pathway or if there is important protein for example, it will be transported to plasma membrane via endocytic pathway. As, these three 1) lysosomal pathway, 2) regulated secretory pathway and 4) endocytic pathway works along in a systemic matter together and requires additional signals for transporting protein to different location. Hence, these are the incorrect answers.

The default secretory pathway, has vesicles (default vesicles) also known as constitutive pathway in which there is no signal or tags that will guide the proteins about their destination. Therefore, in these they directly secrete by fusing with the plasma membrane.

Hence, option 3) default secretory pathway is the correct answer.