selectins
Summary
Selectins are cell-surface adhesion proteins that bind specific carbohydrate groups on other cells' glycoproteins in a Ca²⁺-dependent way. They mediate the weak, transient binding that makes leukocytes roll along blood vessel walls before integrins produce firm attachment.
Detail
Meaning: Selectins are cell adhesion molecules (CAMs) that act as lectins, which are proteins that bind carbohydrates. Their ligands are sialylated, fucosylated oligosaccharides on glycoproteins and glycolipids of a neighboring cell. Binding needs Ca²⁺ and is low affinity with fast on and off rates, so a leukocyte is slowed and rolls instead of stopping. The three types are L-selectin (leukocytes), E-selectin (endothelium), and P-selectin (platelets and endothelium).
On the MCAT: The usual setup is leukocyte extravasation during inflammation. Step 1: selectins give rolling. Step 2: chemokine signals activate integrins on the leukocyte. Step 3: integrins bind ICAMs on the endothelium for firm adhesion. Step 4: the cell squeezes between endothelial cells (diapedesis) into tissue. Questions ask which molecule drives which step, or predict that a selectin defect or blocking antibody leaves leukocytes unable to roll, so fewer reach the inflamed site. Selectins also serve as the standard example of protein–carbohydrate recognition.
Common confusion: Selectins, integrins, and cadherins are often mixed up. Ca²⁺ dependence does not separate selectins from cadherins, because both need it. What settles it is the partner: selectins bind carbohydrates on another cell, cadherins bind other cadherins (homophilic cell–cell junctions), and integrins bind extracellular matrix proteins or ICAMs.
Sources
- Molecular Biology of the Cell
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.