<P> The endoplasmic reticulum is also part of a protein sorting pathway . It is, in essence, the transportation system of the eukaryotic cell . The majority of its resident proteins are retained within it through a retention motif . This motif is composed of four amino acids at the end of the protein sequence . The most common retention sequences are KDEL for lumen located proteins and KKXX for transmembrane protein . However, variations of KDEL and KKXX do occur, and other sequences can also give rise to endoplasmic reticulum retention . It is not known whether such variation can lead to sub-ER localizations . There are three KDEL (1, 2 and 3) receptors in mammalian cells, and they have a very high degree of sequence identity . The functional differences between these receptors remain to be established . </P> <P> Abnormalities in XBP1 lead to a heightened endoplasmic reticulum stress response and subsequently causes a higher susceptibility for inflammatory processes that may even contribute to Alzheimer's disease . In the colon, XBP1 anomalies have been linked to the inflammatory bowel diseases including Crohn's disease . </P> <P> The unfolded protein response (UPR) is a cellular stress response related to the endoplasmic reticulum . The UPR is activated in response to an accumulation of unfolded or misfolded proteins in the lumen of the endoplasmic reticulum . The UPR functions to restore normal function of the cell by halting protein translation, degrading misfolded proteins, and activating the signaling pathways that lead to increasing the production of molecular chaperones involved in protein folding . Sustained overactivation of the UPR has been implicated in prion diseases as well as several other neurodegenerative diseases and the inhibition of the UPR could become a treatment for those diseases . </P>

The smooth endoplasmic reticulum is associated with the production of what