Ritu's Biology Hub

Continued from Part 2

Integrins

• Principal receptors on animal cells for binding extracellular matrix proteins
• Composed of two transmembrane subunits that are noncovalently attached – α, β
• β₁ subunit – Form dimers with at least 12 different α subunits
  • α₅β₁ – Is a fibronectin receptor
  • α₆β₁ – Is a laminin receptor
• β₂ subunit – Present on WBCs enabling them to attach firmly to endothelial cells at sites of infection and migrate out of bloodstream to the infected site. Mediate cell-cell interaction. Ligand involved is ICAM.
• β₃ subunit – On platelets, mediate attachment to fibrinogen. Glanzmann’s disease – deficiency of this subunit
• α₆β₄ subunit – Is an exception that binds to intermediate filaments
• Cells can control integrin-ligand interactions from within and is called as inside-out signalling. This is seen during activation of platelets and when WBCs migrate to tissues from blood
• Platelets activated by either damaged vessels or signal molecules activate β₃ integrins in their plasma membrane. This induces a conformational change in their extracellular domain that enables it to bind fibrinogen. Fibrinogen then aggregates platelets forming clot to stop bleeding.
• T cells bind weakly to its specific antigen on APCs (antigen presenting cells). This triggers intracellular signalling pathways that lead to the activation of β­_2. The activated T lymphocytes then bind strongly to the APCs, prolonging the time of contact to become fully stimulated.

At A Glance