Cell Biology: Cell Surface and Communication Part I

Extracellular Matrix (ECM)

• The extracellular matrix is a jelly of proteins and polysaccharides in which the single cells of the simplest multicellular animals are embedded
• Cells produce and secrete these proteins and polysaccharides, creating their own immediate environment
• Components
• Proteoglycans = a type of glycoprotein that cushions cells and bind a wide variety of extracellular molecules
  
• Collagens = proteins that often form fibers; provide mechanical strength and resilience
  
• Fibronectin = soluble multiadhesive matrix proteins that bind to and cross-link cell-surface adhesion receptors and other ECM components
  
• Functions
• In animals, the extracellular matrix cushions and lubricates cells
• Provides mechanical support to tissues
• Provides a lattice through which cells can move
• Serves as a reservoir for many extracellular signaling molecules that control cell growth and differentiation
  
• Using different combinations of ECM components, the ECM can:
• strengthen a tendon, tooth, or bone
• cushion cartilage
• provide adhesion in most tissues
• provide the cell with environmental cues to know where it is and what it should do
• The Basal Lamina source
• A specialized, tough, sheetlike meshwork of ECM components that form a supporting layer underlying sheetlike cell layers and helps prevent the cells from ripping apart
• Have roles in regeneration after tissue damage and in embryonic development
• Helps organize cells into tissues and guides migrating cells during tissue formation
• Four ubiquitous protein components found in basal laminae
• Type IV collage = trimeric molecules with rodlike and globular domains that form a 2D netowrk
• Laminins = multiadhesive proteins that form a fibrous 2D network with type IV collage and also bin to integrins
• Entactin = rodlike molecule that cross-links type IV collage and lmainin and helps incorporate other components into ECM
• Perlecan = a large multidomain proteoglycan that binds to and cross-links many ECM components and cell surface molecules

• Cell-Matrix Adhesion source
• Adhesion receptors bind to various components of the ECM t0 meditate cell-matrix adhesions
• Responsible for directly or indirectly linking the CAM to the cytoskeleton (actin or intermediate filaments) and to intracellular signaling pathways
• Allows information to be transferred by CAMs and macromolecules in the ECM to which they bind
• Integrins
• Function as adhesion receptors to mediate many cell-matrix interactions
• are heterodimeric integral membrane proteins
• Members of the integrin family play important roles in adhesion and signaling in both epithelia and nonepithelial tissues
• integrins in hemidesmosomes help adhere cells to basal lamina
• some integrins participate in heterophilic cell-cell interactions in some blood cells
• Although they have low affinities for the ligands, binding of hundreds or thousands of integrins firmly anchor cells to the ECM
• Weak interactions also important to facilitate cell migration

Cell Wall source

• Plant Cell Wall
• A laminate of cellulose fibrils in a matrix of glycoproteins that completely coats the outside of the plant cell's plasma membrane
• Serves some of the same functions as the animal cell's ECM (although are composed of entirely different macromolecules and have a different organization)
• Connects cells into tissues
• Signals a plant cell to grow and divide
• Controls the shape of plant organs
• Has roles in controlling the differentiation of plant cells during embryogenesis and growth
• Structure
• Arranged into layers of cellulose microfibrils - bundles of long, linear, extensively hydrogen-bonded polymers of glucose in beta glycosidic linkages
• Cellulose microfibrils are embedded in a matrix of pectin and hemicellulose
• Layers of microfibrils prevent cell wall from stretching laterally
• Permeability of the cell wall is controlled largely by pectins
• No CAM plant homolog
• Adhesive-type proteins in plants are the wall-associat kinases (WAKs) and WAK-like proteins