The ER has many complex proteins and so have a robust system for heat-shock protein assisted folding

The resulting three-dimensional structure is determined by the amino-acid sequence or primary structure (e.g., Anfinsen's dogma). The amino acids interact with each other to produce a well-defined three-dimensional structure, the folded protein (see the right side of the figure), known as the native state. The folding of many proteins begins even during the translation of the polypeptide chain. As the polypeptide chain is being synthesized by a ribosome, the linear chain begins to fold into its three-dimensional structure. At this stage, the polypeptide lacks any stable (i.e., long-lasting) three-dimensional structure (see the left side of the first figure). Each protein exists first as an unfolded polypeptide or random coil after being translated from a sequence of mRNA into a linear chain of amino acids. Via an expeditious and reproducible process, a polypeptide folds into its characteristic three-dimensional structure from a random coil. Protein folding is the physical process where a protein chain is translated into its native three-dimensional structure, typically a 'folded' conformation, by which the protein becomes biologically functional.

Change of a linear protein chain to a 3D structure Protein before and after folding Results of protein folding

The ER has many complex proteins and so have a robust system for heat-shock protein assisted folding