The figure provided below schematically depicts proteins con

The figure provided below schematically depicts proteins containing various types of signal and topo genic sequences Predict the arrangement of each type of protein shown in the figure with respect to the endoplasmic reticulum membrane and lumen.

Solution

Remember two things:

1. Cleavable signal sequence is necessary to enter inside the ER.

2. non cleavable signal anchor sequence will let the protein pass into the lumen but will held it on membrane.

3. Stop transfer sequence will not let the protein pass to the ER lumen.

In the question, the symbol for non cleavable signal anchor sequence and stop transfer sequence is same. It might be due to misprinting. So, I am taking +box as non cleavable signal anchor sequence and just box as stop transfer sequence.

a) just contains the cleavable signal sequence. So, the whole protein will be inside the ER lumen.

b) contains only non cleavable signal anchor sequence. So, the protein will go inside the ER and will be anchored to the inner membrane only.

c) contains cleavable signal sequence. So, this part will cross the membrane but is followed by a stop transfer sequence where the transfer will stop. So, it will be a single pass transmembrane protein.

d) The + sign will face the membrane. Put the first box just inside the membrane and the second box just outside with the intermediate protein inside the membrane. The third box is a stop transfer sequence. So, it will remain outside the membrane. The furth box will be on the outer membrane and it enters from the plus sign. Then last box prevents the entry and will remain on outside membrane.

e) The first box will be inside the membrane. The second box is stop transfer so it will be outside till the third box which will get inside and the last box will be stop sequence. The rest part of the protein remains outside.

So, in nutshell, think like this, the + part goes inside the box remains in the membrane the stop sequence remains outside the membrane.