Final answer:
Selectins are cell-surface receptors with a small cytoplasmic tail, a single transmembrane domain, and a large extracellular domain. The extracellular domain binds to ligands outside the cell, while the transmembrane domain anchors the selectin in the membrane and the cytoplasmic tail interacts with intracellular signaling pathways.
Step-by-step explanation:
Selectins have a small cytoplasmic tail, a single membrane-spanning domain, and a large extracellular domain.
Each cell-surface receptor, including selectins, has three main components: the extracellular domain which is typically the ligand-binding domain, a hydrophobic membrane-spanning region, often called the transmembrane domain, and an intracellular domain inside the cell. The sizes of these domains can vary greatly depending on the type of receptor. In the case of selectins, their primary role involves cell-cell signaling processes.
The extracellular domain is responsible for binding to specific molecules outside the cell, such as ligands, which are typically hydrophilic and cannot penetrate the hydrophobic interior of the plasma membrane. The transmembrane domain allows selectins to be anchored within the cell's membrane, while the cytoplasmic tail interacts with various signaling pathways within the cell.