Clathrin is a protein with an extraordinary structure. It is a trimer with three leg-like subunits.
Structure of clathrin. A: a single molecular complex. B: aggregation of the clathrin molecules at the surface of a coated vesicle. The hexagons are easily detected in the electron microscope (according to R. A. CROWTHER, B. M. F. PEARSE, 1981).
Its extraordinary structure enables the protein to polymerize in a two-dimensional network consisting of numerous hexagons. Strictly speaking forms the aggregate no plane but a bent surface with a convex and a concave side. It is an open question whether the tendency to bend has intramolecular or intermolecular causes. Important is especially the fact that such networks fit with their concave side tightly to membranes, for example to the inner surface of the plasmalemma. The growing network provides the mechanical force to pull the membrane into a bud. This bud is finally pinched off: a clathrin-coated vesicle has been formed.
Coated vesicles are known to exist in a range of plant and animal cells (E. H. NEWCOMB, 1980). They bring extracellular substances into the cell, a process called endocytosis. Depending on whether the vesicles cargo consists of particles or of liquid is it distinguished between phagocytosis and pinocytosis. Within the cell have coated vesicles the chance to fuse with other vesicles, for example with lysosomes. Their content is then digested by the lysosomal enzymes, the clathrin coat is dismantled and available for a new cycle.
Formation of endoplasmatic coated vesicles in the plasma membrane of an isolated rat hepatocyte in culture. Notice the regularly arranged coat at the cytoplasmatic side of the membranes. The second picture shows the fusion of an isolated secretory vesicle with a coated vesicle (arrow). Third picture: isolated coated vesicles (KARTENBECK, Heidelberg)
Scheme of the function coated vesicles have in phagocytosis. Shown is, too, the recycling of the vesicles. Ingested molecules or particles (red) can be digested, kept in storage vacuoles or be released into the cytosol. At the top of the picture: the coated vesicle buds (A, B) and pinches off (C) At the middle of the picture (2): the coated vesicle fuses with an intracellular vesicle, its content is processed. 3, 4: indicated fusion of the now un-coated vesicle with the plasmalemma (membrane recycling). Top left corner (5): insertion of clathrin into the plasmalemma, the prerequisite for the formation of new coated vesicles (according to B. PEARSE, 1980).
If a vesicle pinches off an endomembrane and fuses with the plasmalemma, is its content exported out of the cell, the process is called exocytosis. Exocytotic vesicles are usually not clathrin coated, most of them have no coat at all.
It seems as if microtubuli participate in the intracellular transport of coated vesicles. A number of results gained with the electron microscope indicate that they are in contact with them and / or with protofilaments (= one-dimensional chains, build from tubuline dimers) (M. E. DOOHAN, B. A. PALEVITZ, University of Georgia, Athens, 1980).
© Peter v. Sengbusch - b-online@botanik.uni-hamburg.de