18. Which of the following molecules is involved in Ca²⁺ – dependent cell-cell adhesion?
(1) Calmodulin                      (2) Cadherin
(3) N-CAM                            (4) Calpain

The molecule involved in Ca2+-dependent cell-cell adhesion is (2) Cadherin. Cadherins are transmembrane proteins that require calcium ions for their extracellular domains to mediate strong, specific cell-cell adhesion in tissues such as epithelia and during early embryonic development.​

Introduction: Calcium-Dependent Adhesion Mechanisms

Cell-cell adhesion occurs through a variety of molecules, but cadherins stand out for requiring calcium ions (Ca2+) to function properly. This dependency is fundamental to tissue architecture, cell sorting, and morphogenesis during development.​

Why Cadherin Is Unique

Cadherins are named for their “calcium-dependent adhesion” properties. They have extracellular domains that rely on bound Ca2+ to maintain the correct conformation for adhesive interactions. Removal of calcium disrupts cadherin-mediated junctions, causing cells to lose contact. This feature is critical during embryonic compaction, tissue formation, and maintenance of epithelial integrity.​

Why Other Options Are Incorrect

• Calmodulin: A calcium-binding protein involved in intracellular signaling, not cell adhesion.

• N-CAM: Neural cell adhesion molecule, part of the immunoglobulin superfamily, mediates calcium-independent adhesion.​

• Calpain: A calcium-dependent protease, involved in protein cleavage, not adhesion.

Table: Molecules and Cell Adhesion

Conclusion

• Cadherin is the prototypical molecule involved in Ca2+-dependent cell-cell adhesion, essential for tissue formation and stability.​

• The other molecules listed do not mediate calcium-dependent cell-cell adhesion, highlighting the unique structural and functional significance of cadherins.