Question

Which types of cell junction contain actin filaments? Which contains intermediate filaments? Which contain integrins?  

Answer 1

1. cell junctions contain actin filaments

zonula adherens - which contain actin filaments

macula adherens (desmosomes) which contain intermediate filaments.

Cells have three types of filaments

• microfilaments - actin filaments (red in the diagram)
• intermediate filaments (blue in the diagram)
• microtubules (green in the diagram).

 

Integrins are a large family of heterodimeric transmembrane glycoproteins that attach cells to extracellular matrix proteins of the basement membrane or to ligands on other cells. Integrins contain large (?) and small (?) subunits of sizes 120-170 kDa and 90-100 kDa, respectively. Some integrins mediate direct cell-to-cell recognition and interactions. Integrins contain binding sites for divalent cations Mg²⁺ and Ca²⁺, which are necessary for their adhesive function. Mammalian integrins form several subfamilies sharing common? subunits that associate with different? subunits.

?₂ integrins are exclusively expressed on leukocytes and undergo a conformational change encompassing the phosphorylation of the ? subunit upon activation. However, this phosphorylation is neither necessary nor sufficient for conformational activation. The activation status is controlled by the GFFKR site immediately adjacent to the transmembrane domain of the alpha chain. A structural model of a ?2 integrin (Harris et al. 2000) is suggested by site-directed mutagenesis and truncation mutants (link to Harris, E.S., McIntyre, T.M., Prescott, S.M., and Zimmerman, G.A. The leukocyte integrins. J.Biol.Chem. 275(31):23409-23412, 2000) ?₂ integrins include 4 different heterodimers CD11a/CD18 [Lymphocyte Function-Associated Antigen-1] (LFA-1) the predominant ?₂ integrin, CD11b/CD18 (Mac-1) exclusive to granulocytes and monocytes, CD11c/CD18 (p150,95), and CD11d/CD18. A mutation in the gene encoding the ?₂ (CD18) molecule results in a genetic disorder, LAD, Leukocyte Adhesion Deficiency. LAD patients have recurrent bacterial infections due to the inability to effectively recruit granulocytes in response to infections.

The most important member of the ?₁ integrin subfamily on leukocytes is Very Late Antigen-4 (VLA-4, CD49d/CD29, ?4 ?₁). VLA-4 binds to its ligand VCAM-1, and is chiefly responsible for lymphocyte adhesion to vascular endothelium and leukocyte recruitment to the inflamed area.