Cell Biology

... from active transport to vesicles


The cells of most eukaryotic species adhere to one another in co-operative multicellular organisms, whereas prokaryotes are unicellular organisms even though some species form colonies. Cell adhesion relies upon specialized transmembrane cellular adhesion molecules (CAMs) that usually extend from the intracellular space to the extracellular space where they may bind to other cell membranes or to the extracellular matrix. Within the intracellular domain, adhesion proteins adhere directly to, or are coupled to, the cell's cytoskeleton. Cadherin (calcium dependent adhesion) molecules are normally coupled by special linking proteins – catenins – to the cytoskeleton.

Intermediate filaments are about 10 nm diameter and provide tensile strength for the cell and connect adjacent cells through desmosomes (macula adherens).
cytoskeleton : diagram . desmosome : tem_desmosome : diagram . tight junction : diagram . gap junction : image - cytoskeleton : image_cytoskeleton : diagram - mechanism of ciliary motility Џ beautiful Flash 8 animation - Inner Life of the Cell, which shows the sequelae of adhesion-signaling, and Interpretation: Inner Life of the Cell Џ

 Cell Adhesion Molecules  Second Messengers  Cell signaling

There are several families of adhesion proteins, each with specific ligands of the same type (homophilic) or a different type (heterophilic).
1. integrins with heterophilic attachments to different (hetero) ligands in the extracellular matrix
2. selectins with heterophilic attachments to carbohydrate ligands
3. Ig superfamily proteins with selectins with heterophilic attachments to: a) integrin ligands, and b) Ig superfamily proteins of a different (hetero) type, and
homophilic attachments to the Ig superfamily proteins of the same (homo) type
4. cadherins with homophilic attachments to cadherins of the same type, or by way of catenins (right - click to enlarge, description), to the cytoskeleton

Cell adhesion is important in:
1. maintaining contact within solid tissue
2. embryogenesis (morphogenesis)
3. migration of single cells such as leukocytes within multicellular organisms
4. maintaining contact between neuronal elements
5. virulence of virions and bacteria

Some signaling molecules act as adhesion receptors, and cluster in focal adhesions upon ligand binding. (Rho protein). A variety of integrins, which are transmembrane heterodimeric adhesion receptors are known to support adhesion-dependent growth factor-activation of MAP kinase. Focal adhesions are rich in tyrosine phosphorylated proteins, coupling cell adhesion to signal transduction pathways in the cell. Various adhesion receptors, such as integrin, are closely linked to protein kinases and phosphatases. Grb2 links focal adhesion kinase (FAK) to the Ras pathway when Grb2 is phosphorylated after binding to FAK. The 85 kDa subunit of the PI 3-kinase is also phosphorylated after binding to FAK. Thus, FAK is a key component in the assembly of focal contact structures that influence cytoskeletal organization and signal transduction.

Engagement of ICAM-1, a member of the immunoglobulin supergene family (Ig), has been documented to activate specific kinases through phosphorylation, resulting in activation of transcription factors, increased cytokine production, increased cell membrane protein expression, production of reactive oxygen species, and cell proliferation.

cadherins : Ig superfamily proteins : integrins : selectins : ICAMs :  Cell Adhesion Molecules
 Second Messengers  Cell signaling .

Cells form a variety of intermembranous junctions: adherens junctions, desmosomes (plasmodesmata), focal contacts, gap junctions, hemidesmosomes, tight junctions.

cytoskeleton : diagram . desmosome : tem_desmosome : diagram . tight junction : diagram . gap junction : image - cytoskeleton : image_cytoskeleton : diagram - mechanism of ciliary motility :

• A • adhesion • C • cell membranescellular adhesion moleculescellular signal transductioncentrioleschemotaxischloroplastcilia & flagellacommunicationconcentration gradientscytokine receptorscytoplasmcytoskeleton • E • energy transducersendoplasmic reticulumendosomesexosome • F • flagella & cilia • G • Golgi apparatusGPCRs • H • hormones • I • ion channels • L • lysosome • M • meiosismicrotubulesmitosismitochondrion • N • Nitric Oxideneurotransmissionneuronal interconnectionsnuclear membranenuclear pore • P • pinocytosisproteasomepumps • R • receptor proteinsreceptor-mediated endocytosis • S • second messengerssignaling gradientssignal transductionspindlestructure • T • transporttwo-component systems • V • vacuolevesicle

. . . developing since 10/06/06