Cell Walls, the Extracellular Matrix, and Cell Interactions (part 1)

Transcription

1 14 Cell Walls, the Extracellular Matrix, and Cell Interactions (part 1)

2 Introduction Many cells are embedded in an extracellular matrix which is consist of insoluble secreted macromolecules. Cells of bacteria, fungi, algae, and plants are surrounded by rigid cell walls. Extracellular matrices have tough fibrous proteins embedded in a gel-like polysaccharide.

3 Introduction

4 Introduction The extracellular matrix provides structural support to cells and tissues and plays important roles in regulating cell behavior. Interactions between cells are key to the organization and communication of cells. Cell walls determine cell shape and prevent cells from swelling and bursting as a result of osmotic pressure.

5 Cell Walls Bacterial cell walls determine the characteristic shapes: Rod-shaped (such as E. coli) Spherical (such as Staphylococcus) Spiral-shaped (e.g., the spirochete Treponema pallidum, which causes syphilis)

6 Cell Walls Bacteria are divided into two classes, based on cell-wall structure: Gram-negative bacteria Gram-positive bacteria

7 Figure 14.1 Bacterial cell walls

8 Cell Walls The principal component of all bacterial cell walls is peptidoglycan: linear polysaccharide chains cross-linked by short peptides. Cell-wall structure makes bacteria vulnerable to some antibiotics. Penicillin inhibits the enzyme that forms the crosslinks, preventing cell-wall synthesis and bacterial growth.

9 Figure 14.2 The peptidoglycan of E. coli

10 Cell Walls Cell walls of eukaryotes are composed of polysaccharides. In fungal cell walls, the polysaccharide is chitin, which also forms the exoskeletons of arthropods. Chitin is a linear polymer of N-acetylglucosamine residues. Cell walls of algae and higher plants are mostly cellulose: a linear polymer of glucose, often with more than 10,000 glucose monomers.

11 Figure 14.3 Polysaccharides of cell walls (Part 1)

12 The Extracellular Matrix and Cell-Matrix Interactions Most animal cells are embedded in an extracellular matrix. Basal laminae: thin layers on which epithelial cells rest.

13 The Extracellular Matrix and Cell-Matrix Interactions Extracellular matrix is most abundant in connective tissues. Connective tissues (loose connective tissue, bone, tendon, and cartilage) consist mostly of extracellular matrix with cells distributed throughout.

14 The Extracellular Matrix and Cell-Matrix Interactions Different types of matrices have different amounts of each component: Tendons - high proportion of fibrous proteins. Cartilage - high level of polysaccharides that form a compression-resistant gel. Bone matrix is hardened by calcium phosphate crystals.

15 The Extracellular Matrix and Cell-Matrix Interactions The major structural protein is collagen. Collagens form triple helices. Triple helix domains consist of repeats of the amino acid sequence Gly-X-Y.

16 The Extracellular Matrix and Cell-Matrix Interactions Glycine is the smallest amino acid. It allows polypeptides to pack closely together. Hydroxyproline is formed by hydroxylation of proline. Hydroxyl groups are thought to stabilize the triple helix by forming hydrogen bonds.

17 The Extracellular Matrix and Cell-Matrix Interactions Type I collagen is the most abundant type. It forms collagen fibrils in which the triple helical molecules form regular staggered arrays. Assembly of fibrils occurs outside the cell from soluble precursor procollagens. Covalent cross-links between side chains of lysine and hydroxylysine residues help strengthen the fibrils. Fibrils can come together to form collagen fibers, which can be several μm in diameter.

18 Figure Collagen fibrils

19 The Extracellular Matrix and Cell-Matrix Interactions Some types of collagen do not form fibrils. Basal laminae are mostly type IV collagen, but also VI and XVIII. All are network-forming collagens. The Gly-X-Y repeats are interrupted by short nonhelical sequences, making them more flexible. They form 2-D cross-linked networks instead of fibrils.

20 Figure Type IV collagen

21 The Extracellular Matrix and Cell-Matrix Interactions Connective tissue with elastic fibers is common in organs that stretch and return to shape, such as the lungs. Elastic fibers are made of elastin which is crosslinked into a network. It behaves like a rubber band.

22 The Extracellular Matrix and Cell-Matrix Interactions Extracellular matrix gels are formed from polysaccharides called glycosaminoglycans (GAGs) that are repeating units of disaccharides. Except for hyaluronan, the sugars are modified with sulfate groups.

23 The Extracellular Matrix and Cell-Matrix Interactions The addition of sulfate groups make GAGs highly negatively charged. They bind positively charged ions and trap water molecules to form hydrated gels. Hyaluronan is the only GAG that is a single long polysaccharide chain. All of the other GAGs are linked to proteins to form proteoglycans.

24 The Extracellular Matrix and Cell-Matrix Interactions Proteoglycans interact with hyaluronan to form large complexes in the extracellular matrix. Aggrecan, the major proteoglycan of cartilage, has about 100 chains of chondroitin sulfate attached to a core protein.

25 The Extracellular Matrix and Cell-Matrix Interactions Matrix adhesion proteins link matrix components to one another and to cell surfaces. Fibronectin is the main adhesion protein of connective tissues, which is often cross-linked into fibrils. It has binding sites for both collagen and cell. Fibronectin is recognized by cell surface receptors and is responsible for the attachment of cells to the extracellular matrix.

26 Figure Structure of fibronectin