Actin

Introduction to Actin

Collagen, found in various extracellular matrices, is the most abundant protein in the body. Collagens provide critical structural support to various tissues and also provide various cellular signals.

Actin Structure

There are over 20 types of collagen, but a typical collagen molecule is a long fibrous structure with three polypeptide chains wound around each other to form a triple-helix. These chains, which are more extended than alpha helices, are held together by H-bonds.

Collagen primary structure, which is usually around 1000 residues, follows a repeating sequence of Gly-X-Y, where X is often hydroxyproline and Y is often hydroxylysine. Glycine residues allow tight packing of the triple helix, while hydroxyproline stabilizes interchain H bonds.

Actin as a Structural Protein

Collagens are made by stromal cells - fibroblasts, osteoblasts, or chondrocytes.

Hydroxylation of proline or lysine residues occurs in the ER and requires oxygen and the reducing agent ascorbic acid (vitamin C) to occur. Vitamin C dieficiency results in scurvy, which often manifests with bruising due to capilary fragility and blood extravasation.

Actin as a Molecular Machine Protein

Collagens can be organized into three groups:

1. Fibril-forming collagens, such as Types I, II, and III, are rope-like and provide high tensile strength.

Type I collagen is found in tendon, bone (check) and ...

Type II collagen is restricted to cartilage

Type III collagen is common in distensible tissues such as blood vessels.

2. Network-forming collagens form a three-dimensional mesh work.

Type IV is a major constituent of the basement membrane, providing support to cells and functioning as a semi-permeable membrane.

3. Fibril-associated collagens bind to collagen fibrils and link them to one another and other components of the ECM.