In the ER the proteins acquire their three dimensional structure with the help of specific chaperon proteins. The chaperons increase the rate of folding while prevent the aggregation and misfolding. In the ER lumen the attached oligosaccharide chain will be further modified. The proper S-S bonds are formed in proteins with the help of protein disulphide isomerases, and the peptidyl-prolyl isomerases help the cis-trans rearrangements of peptide bonds. As soon as the proper three dimensional structure is achieved, the glucose is cleaved off and the protein is wrapped into specific vesicles and transported to the Golgi system. The assembly of the multimer proteins happens in the ER as well. The presence of glucose indicates that the protein is not ready for transport into the Golgi system thus the ER chaperons the calnexin and calreticulin bind the glucose containing proteins, helping their folding. In the ER, there is an efficient quality control system which prevents the transport of partially folded or misfolded proteins. When the misfolded proteins cannot be fixed, the ER associated degradation (ERAD) system targets them back to the cytosol where they will be ubiquitinated and degraded by the proteasome (Figure 3.7.).