32. There are many superfamilies of adhesion proteins, which play a central role in cell-cell adhesion in animal. lg superfamily proteins are one such adhesion proteins. Which one of the following statements about lg superfamily proteins is INCORRECT?
    (1) The white blood cell proteins recognized by endothelial cell integrins are called ICAM
    (intercellular cell adhesion molecule) or VCAM (vascular cell adhesion molecules)
    (2) These are called lg superfamily because they contain one or more extracellular lg-like domains that are characteristic of antibody molecules.
    (3) ICAM and VCAM mediate heterotrophic binding to integrin, whereas NCAM (neural cell adhesion molecule) mediates homotrophic binding.
    (4) They contain large quantities of sialic acid which inhibit adhesion by charge-based repulsion contributing to fine tuning of cell-cell adhesion.

The INCORRECT statement about immunoglobulin (Ig) superfamily proteins is:

(4) They contain large quantities of sialic acid which inhibit adhesion by charge-based repulsion contributing to fine tuning of cell-cell adhesion.

Explanation:

• Statement (1) is true: White blood cell proteins recognized by endothelial cell integrins are indeed ICAMs (intercellular adhesion molecules) and VCAMs (vascular cell adhesion molecules), both members of the Ig superfamily involved in immune cell adhesion and trafficking.​

• Statement (2) is true: Ig superfamily proteins contain one or more extracellular Ig-like domains characteristic of antibody molecules (immunoglobulins). These domains feature a sandwich-like structure with beta sheets stabilized by disulfide bonds.​

• Statement (3) is true: ICAMs and VCAMs mediate heterophilic binding to integrins, while NCAM (neural cell adhesion molecule), another Ig superfamily member, mediates homophilic binding, facilitating diverse cell adhesion mechanisms.​

• Statement (4) is incorrect: While sialic acid residues commonly modulate adhesion via charge effects on certain glycoproteins, it is not a defining characteristic that Ig superfamily proteins contain large quantities of sialic acid that inhibit adhesion. The Ig superfamily adhesion is primarily mediated by their Ig-like domains and specific protein-protein interactions rather than charge-based repulsion by sialic acids.​

Summary Table

Conclusion

The incorrect statement is (4). Immunoglobulin superfamily proteins do not generally contain large amounts of sialic acid that inhibit adhesion by charge-based repulsion; their adhesive functions depend mainly on Ig-like domain-mediated protein interactions.