Integrins

Integrins are transmembrane adhesion receptors composed of alpha and beta glycoprotein subunits that connect cells to the extracellular matrix. They bind specific ligands like the RGD sequence in fibronectin, serving as mechanical linkers between the cytoskeleton and ECM. This connection enables cells to sense their environment and facilitates cell motility, growth, and survival through bidirectional signaling across the plasma membrane.

Integrins are heterodimeric transmembrane proteins with extracellular and intracellular domains. The alpha subunit contains a seven-bladed beta-propeller head domain while the beta subunit has an I-like head domain; together they bind ligand. Two leg-like structures extend from these head domains, and the intracellular C-terminal tail of the beta subunit interacts with actin filaments through adaptor proteins like talin and paxillin.

Mammalian integrins are formed by combinations of 18 different alpha-chain genes and 8 different beta-chain genes, resulting in 24 distinct integrin types in vertebrates. Each combination creates integrins specific to different tissues and functions. This diversity allows cells to recognize and respond to various extracellular matrix components and ligands.

Integrins bind to specific amino acid sequences in extracellular matrix proteins, particularly fibronectin, which connects proteoglycans and collagen within the ECM. The integrin's large N-terminal extracellular domains recognize these ligands, anchoring cells to the matrix network. This interaction allows cells to maintain structural stability and communicate with their surrounding environment through the cell matrix response to mechanical forces.

Integrins exist in active and inactive conformational states that regulate their ligand-binding capacity. In the active form, they function as mechanical linkers connecting the cytoskeleton to the extracellular matrix. The transition between these states is controlled by intracellular signaling affects focal adhesions, enabling cells to dynamically regulate adhesion strength and respond to environmental signals.

Integrin's intracellular C-terminal tail interacts with actin filaments in the cytosol through adaptor proteins such as talin and paxillin. These proteins bridge the gap between the transmembrane integrin and the cytoskeletal network, enabling force transmission. This connection allows cells to translate mechanical signals from the extracellular matrix into intracellular responses that affect cell behavior.

Integrins transmit signals bidirectionally across the plasma membrane with the help of effector proteins, allowing communication between the extracellular matrix and the cell interior. Ligand binding to integrin's extracellular domains triggers conformational changes that activate intracellular signaling cascades. This two-way communication enables cells to respond to external mechanical and chemical cues while simultaneously sending signals outward.