Monday, December 14, 2009

#12 ER lumen protein

GRE-ST question number 54 is about ER proteins and functions.
The KDEL sequence, found on luminal proteins of the ER, is responsible for
(A) translocation of proteins into the ER lumen
(B) insertion of proteins into the membrane of the ER
(C) quality control in the ER
(D) recognition by signal peptidase of the signal sequence
(E) retrieval of ER luminal proteins from the Golgi

Show answer
This question is just seems to be hard. Just google and you'll probably find this wikipedia article on ER, where KDEL function is described clearly.
Point is that there are two types of proteins in ER lumen: those which should be transported further (to Goldgi) and those which work inside ER (e.g. linking oligosaccharides to proteins). So there should be some anchor for that "stable" proteins and it links to KDEL sequence on the end of those proteins. And because they cannot be fixed any way (or it would be very, very energetically unfavourable) they are transported back from Golgi to ER by specific KDEL-recetors.
So right answer is E.