Clathrin

Clathrin is a protein that is involved in the invagination of the cell membranes and the formation of vesicles (especially in the receptor -dependent endocytosis ). After pinching off, the clathrin the Stachelsaumbläschen ( clathrin - coated vesicles ) ATP - dependent distance ( " uncoating " by the uncoating ATPase ).

Clathrin is an unusually structured protein. It is a hexamer of three heavy and three light subunits, of which there are two isoforms. The subunits are arranged in the form of a tripod, in the form of triskelions. Thus, it is possible to form a two-dimensional network, which is composed of hexagons and pentagons. It is generated not a flat plane, but rather a construct comprising a convex and a concave side. The concave face is always facing the membrane.

The clathrin vesicles have diameters of 50 to 100 nm and are covered by a framework of 12 mostly penta-and hexagons 8 from a total of 36 triple skeletons of clathrin. This fibrous protein composed of a heavy chain ( 180,000 daltons [ Da] ) and a light chain ( 35,000 to 40,000 Da). In addition, the 900 amino acid long dynamin that can bind and hydrolyze GTP is, also in Vesikelmantel. Between the clathrin and the outer vesicle membrane, so-called Adaptorproteinkomplexe find (german adapter protein complexes ), abbreviated AP complexes. These complexes, AP- 1 to AP -4 ( ≈ 340,000 Da), composed of four different polypeptides, that are heterotetramers. They bind to the heavy chains of the Clathrins, membrane lipids and membrane proteins and thereby mediate the clathrin- binding to membranes. AP-2 mediates the formation of clathrin - coated vesicles (CCV ) at the plasma membrane. AP -1 mediates CCV formation at the trans - Golgi network, the last compartment of the protein synthesis apparatus in secretory transport.

Clathrin and membrane curvature

Undergraduate textbooks of cell biology designate clathrin as the protein which is responsible for the curvature of the membrane during vesicle formation. It was originally believed that the assembly of the proteins to the clathrin triskelion - structure, the membrane in the curved state of "forces". Recent findings show, however, that other factors play an important role for the effective curvature of the membrane in vivo. This includes, for example, the activity of ATP-dependent Flippasen producing a locally increased asymmetry of the phospholipid composition of the membrane. The intrinsic property of certain phospholipids to curve membranes, leads to the protuberance. Also involved in the shell formation small GTPase Arf has an influence on the membrane curvature. Therefore, it is the interplay of several complex factors that enable effective formation of clathrin - coated vesicles.