ABSTRACT
The mere assembly of amino acids into a polypeptide is not sufficient for the formation of an active protein. The newly synthesized polypeptide must also acquire a specific conformation and in many cases selected amino acid residues must undergo chemical modification. The lumen of the Endoplasmic Reticulum (ER) has a very high concentration of protein-30 to 100 mg/ml. Most of the proteins in this fluid are permanent residents of the ER. Nascent polypeptides appear to cross the ER membrane in an essentially unfolded state. In most cases, the subsequent folding is completed as soon as the polypeptide has emerged in the lumen of the ER. Some secretory proteins consist of different polypeptides coded for by more than one gene. Most of the proteins belonging to the a-macroglobulin family have an internal thiol ester whose function is to form a link to other proteins.
