|Section 4 Index||Index to Course Material|
The destination of proteins synthesized by ribosomes obviously depends on their function. The following applies to eukaryotic cells:
Refer to the page on transport across the E.R. membrane for a description of how proteins destined for secretion are "tagged" so that they enter the endoplasmic reticulum.
Mutation studies have indicated that once synthesized, the secreted proteins are situated in various organelles and vesicles in the following order:
The coated vesicles which bud from the E.R. have their membranes enclosed by a polyhedral clathrate ("cage-like") structure, consisting of an association of triskelions. The triskelions are flexible, "three-legged" protein complexes composed of the protein clathrin and its associated proteins. The sides of the polyhedron are formed by overlapping "legs", and are about 150Å long. The faces of the framework are pentagonal (always 12) and hexagonal (variable number)- similar constraints of symmetry apply as in the "spherical" viruses.
The condensing vesicles, also coated, bud off from the Golgi apparatus and are converted to mature secretory vesicles by concentration of their contents.
Some types of cells continuously secrete proteins in these vesicles, e.g serum proteins and collagens, while in others the proteins are stored in vesicles awaiting a hormone signal to trigger their exocytosis. The secretory vesicles of pancreatic exocrine cells are called zymogen granules as the proteins in them are the inactive forms (zymogens) of the digestive enzymes.
Proteins are glycosylated in the rough E.R. Further modifications of the carbohydrate chains occurs in the Golgi apparatus. This process serves to label various types of proteins to direct them to their correct destinations (see below). A page on glycosylation of proteins will follow.
Other non-cytoplasmic proteins are synthesized by the ribosomes on the rough E.R. besides those which are destined for secretion. These include integral membrane proteins, not only in the plasma membane but also in the membranes of other organelles such as the smooth E.R. Lysosomal enzymes are also synthesized by the rough E.R. ribosomes. (Here are notes on lysosomes.)
The oligosaccharide added to lysosomal enzymes in the rough E.R. lumen is the same as that which is added to secretory proteins. However, one or more mannose residues of the lysosomal enzymes are phosphorylated. This acts as a signal which leads to the enzymes eventually entering the lysosomes. This phosphorylation occurs in two steps, catalyzed first by N-acetylglucosamine phosphotransferase, and then by a phosphodiesterase, giving a mannose 6-phosphate residue. These enzymes are located in the Golgi apparatus.
The lumenal face of the Golgi membrane has a receptor for mannose 6-phosphate. It is thought that regions of this membrane including the receptors and bound glycosylated enzyme bud off to form transport vesicles which carry their contents to sorting vesicles. Sorting vesicles are acidic (pH 5), which is not favourable for binding of the mannose-6-phosphate receptor and ligand. The released lysosomal enzymes then have their mannose 6-phosphate residues dephosphorylated, so that the enzyme cannot re-bind to the receptor. Vesicles containing the enzymes but devoid of the receptor then bud from the sorting vesicle and fuse with lysosomes.
Read about the role of cathepsin in Baldwin et al. (1993).
cathepsin D 1lya (585Kb) [Bbk|BNL|ExP|Waw|Hal] cathepsin D complexed with pepstatin 1lyb (601Kb) [Bbk|BNL|ExP|Waw|Hal]; two enzymes in the asymmetric unit in each case. Both enzymes consist of two chains, due to post-translational cleaving.
Last updated 27th Nov'96