29.1: Cell Adhesion Molecules - Types and Functions
Cell adhesion molecules (CAMs) are pivotal to multicellularity and the coordinated functioning of tissues and organ systems. They enable physical interactions between cells and provide mechanical strength to tissues. They also function as receptors for signal transmission across the plasma membrane. The CAMs are broadly classified into four families - integrins, cadherins, selectins, and immunoglobulin-like CAMs (IgCAMs).
The Integrin family of proteins is primarily involved in a cell’s interaction with its surrounding matrix. However, some integrins can bind CAMs on another cell’s surface and participate in direct cell-cell interactions. For example, the integrins on immune cells bind IgCAMs expressed on the vascular endothelium.
Cadherins are a superfamily of calcium-dependent glycoproteins primarily involved in establishing strong cell adhesions. They compose the adherens junctions and desmosomes in tissues, such as the epithelium.
Selectins and IgCAMs are involved in transient cell interactions and participate in directing cells towards target sites. For example, they help in the selective recruitment of lymphocytes to the secondary lymphoid organs.
CAM and Multicellularity
CAMs are found across virtually all multicellular organisms - from sponges and simple nematodes to complex invertebrates and vertebrates. The complexity of the cellular interactions, and therefore of the CAMs, increases with the complexity of the organisms. For example, while the fruit fly D.melanogaster has approximately 500 genes involved in cell adhesion, complex vertebrates like mammals have over a thousand genes that code for different types of CAMs.