Cell Biology--Section III

Dr. Carol Carter

Office Hour: Wednesdays 2-3

Room 314, Sci. Bldg. 1

carterm@pdx.edu

“Cell Ecology”
Interactions between cells and their environment.

• Extracellular space

• Interactions between cells

• Sealing the extracellular space

• Gap junctions and plasmodesmata

• Cell walls

Tissue Organization
in animals

• Connective

• Epithelial

• Nervous

• Muscle

All are associated with ECM, but connective has high ratio of ECM to cells

Extracellular Matrix (ECM)

• Defined: the organized material beyond the plasma membrane.

• Organization by physical, enzymatic factors:

self assembly--free energy changes

binding specificity

pH, redox

enzyme activity

hormones

force fields (mechanical, electrical)

(The important role of
carbohydrates in cell-cell
interactions)



“The information-carrying potential of oligosaccharides is far greater than that of proteins and nucleic acids of equivalent molecular weight, and their presence on cell surfaces and on many proteins suggests an importance which was previously unrecognised. Oligosaccharides are now thought to mediate cell-cell recognition, including the infection of cells by bacteria and viruses, moderate the behaviour of enzymes and other proteins, and play a variety of rôles in the immune response.”

(Fig. of amphiphilic hexose not in text)

An overview of the macromolecular organization of ECM and the plasma membrane in some animals
(fig 7.5)

Some components of the ECM of animals: collagen, fibronection, proteoglycan. (Fig 7.5)

Fibronectin
Fig. 7.5.
(Fig. Of fibronectin gene not in text)

• Evolutionarily conserved protein

• Has functional domains; interacts differentially

• Has arg-gly-asp (RGD) sites

• Tissue variable forms from differential RNA splicing

Collagen
Fig 7.6

• Fibrous protein, abundant.

• Gene family; allows variations per tissue

• Secreted as procollagen, enzyme cleaves, then self-assembly into triple helix

• Fibroblasts may organize

• Tethered by fibronectin, integrin, actin

• Examples: tendons (fibrillar), cornea (crossed), basement membrane (random)

Proteogylcans
Fig. 7.9

• Protein core with glucosamines (GAG) side chains.

• GAGs neg. charged

• GAGs bind cations

• Cations bind water

• _H O _H = gel

• If water can’t flow it resists compression.

• Therefore function of proteoglycans is…..?

Cell-Cell Interactions
Fig 7.19

• Depends on distinguishing self, nonself

• Mediated by recognition factors

• selectins, Ig, integrins, cadherins (all glycoproteins)

Integrins, Fig. 7.5

• Integral gycoprotein- spans membrane. (cytoplasmic and extracellular head)

• Subunits: ", $-- not covalently bound

• Gene family for each ", $ unit--specificity, 22 known

• Inside/outside communication

• Requires Ca

• Binds to arginine/glycine/ aspartate residues (RGD)

• Conserved among animals.

Selectins, Fig. 7.20

• Transmembrane molecule

• Glycoproteins

• Lectins (binds to specific carbohydrate)

• Requires Ca ion

• Mediates transient interactions with blood

Immunoglobulin Superfamily (IgSF)
Fig. 7.21

• Glycoproteins

• Members of SF share similar domains (aa sequence, conformation)

• Most are antibodies (immune response)

• Others have generalized cell adhesion or neural development functions

• Can link with integrins

The immunoglobulin superfamily (IgSF) is the largest known group of related proteins. In a single species, the IgSF includes hundreds of members believed to have arisen from a single primordial Ig-like domain during evolution of multicellular metazoans (Williams and Barklay, 1988; Doolittle, 1995). (Fig. 7.21)

Cadherins
Fig. 7.22

• Family of glycoproteins

• Mediate Ca++ dependent cell adhesion

• Transmit signals from ECM to cytoplasm

• Self recognition (same type of cell)

Cadherins, con’t
Fig 7.27a

• In humans, 40 different kinds known

• Most variety found in brain

• Holds epithelial-derived tissues together (e.g., kidney, skin)

• Ca++ integrated between repeats--holds rigid

• Zips together with like cadherins from another cell

• Bound to cytoskeleton

Sealing Extracellular Space

• In animals “tight junctions” isolate regions of extracellular space

• In plants, impregnation of matrix with solids isolates regions.

• Reason: to control the pathway of diffusion of solutes

Tight Junctions, Fig. 7.30a

• Membrane proteins shared or conjoined.

• Forms ring around cells

• Solutes are restricted in ECM.

• Diffusion is restricted within plane of membrane.

• Common in epithelial tissues

Gap Junctions, Fig. 7.32

• Allow intercellular communication

• Found in verts and inverts, but different proteins

• “Connexons” are found in verts.

• Allow direct channel between cells

• Ion exchanges, etc.

The ECM of Plant Cells--plant cell walls
Fig. 7.35

• Cellulose microfibrils - give strength, shape

• Hemicellulose - cross links

• Pectins: neg. charged - gel, glue

• Glycoproteins - allow changes, recognition, and ?

• Other polymers: lignin, suberin, wax

Function of Cell Walls

• Support

• Protection

• Translocation of water, ions, nutritive substances, hormones

• Specializations can direct transport --lignin, suberin, wax

Wall Specialization: Casparian Strip
(Fig. not in text)

• An impervious strip (suberin) within cell walls of cells surrounding the root vascular tissue. Directs solutes from apoplast to symplast.

Plasmodesmata, (Fig. 7.34)

• Symplastic channel across cell walls between cells

• ER tube goes through

• Cytoplasmic continuum

• amino acids, other charged molecules can pass through; some viruses regulate by “movement protein”

Cellulose Synthesis, Fig. 7.37b

• Cellulose synthase rosettes extrude cellulose polymer.

• Cellulose polymer joins with others to form cellulose microfibril.

• Orientation is related to microtubules

Concept Summary

• Carbos give specificity by conformation

• Glycoproteins carry carbos, bind, allow communication

• Cations (especially Ca++) regulate, bridge, form gels,

• ECM organized by physical factors (e.g. collagen)

• ECM supports, protects, allows cell recognition, interaction

• Fibronectin binds to many types of molecules- specificity by RNA splicing

• Cell-cell interactions mediated by glycoproteins, e.g. integrins, selectins, IgSF, cadherins-- specificity by gene families, gycomoieties

• Tight junctions, Casparian strips-- partition solutes

• Plasmodesmata/ Gap junctions allow cytoplasm continuum

• Integral proteins (cellulose synthase, connexons, integrins) allow inside/out communications & connections