A sorting signal (targeting signal) is the part of a protein that allows it to be incorporated into a nascent transport vesicle. The vesicle moves the protein from one compartment to another. Sorting signals consist of either a short sequence of amino acids or a covalent modification.

Dolichol is a lipid that consists of a long chain of isoprenoid units and is present in the membrane of the rough endoplasmic reticulum. It is part of the precursor in the synthesis of N-linked oligosaccharides. An oligosaccharide is assembled onto dolichol via a pyrophosphoryl linkage, then transferred to particular asparagine residues of a nascent polypeptide.

A high mannose oligosaccharide is an N-linked oligosaccharide that contains N-acetylglucosamine linked only to mannose residues. It is covalently added to transmembrane proteins in the rough endoplasmic reticulum and is trimmed and modified in the Golgi apparatus.

A complex oligosaccharide is an N-linked oligosaccharide that is made during transit through the Golgi apparatus. Mannose residues are trimmed from the high mannose precursor in the rough endoplasmic reticulum and cis Golgi, and other sugars are added by enzymes in the medial and trans Golgi cisternae to form a complex oligosaccharide.

The inner core is an intermediate in the synthesis of N-linked oligosaccharides. It is produced upon the removal of mannose residues from a high mannose oligosaccharide in the cis Golgi and is resistant to degradation by endoglycosidase H.

The terminal region is the part of an N-linked oligosaccharide that consists of all the sugar residues added subsequent to formation of the inner core.

  • A major function of the ER and Golgi is to glycosylate proteins as they pass through the system.
  • N-linked oligosaccharides are initiated by transferring the saccharide from the lipid dolichol to an asparagine of the target protein in the ER.
  • Sugars are trimmed in the ER to give a high mannose oligosaccharide.
  • A complex oligosaccharide is generated in those cases in which further residues are added in the Golgi.