Gag

Glycosaminoglycans (GAGs) are a family of long, unbranched polysaccharides formed by repeating disaccharide units that are typically sulfated and carry a high negative charge. They are major constituents of the extracellular matrix and cell surfaces, where they help regulate hydration, elasticity, and filtration. Most GAGs are covalently linked to core proteins to form proteoglycans, with the exception of hyaluronic acid, which exists as a free, non-proteoglycan polymer.

The major sulfated GAGs include chondroitin sulfate, dermatan sulfate, keratan sulfate, heparan sulfate, and the highly sulfated heparin. Hyaluronic acid is non-sulfated and does not form proteoglycans. GAGs differ in disaccharide composition, sulfation patterns, and tissue distribution. They are synthesized in the Golgi apparatus and degraded in lysosomes by specific hydrolases; abnormalities in degradation cause lysosomal storage diseases known as mucopolysaccharidoses.

Functions of GAGs are diverse and include providing a hydrated, viscoelastic extracellular matrix; binding and modulating growth factors, cytokines, and enzymes; influencing cell adhesion, migration, and signaling; and contributing to the lubrication of joints and the transparency of the eye. Dysregulation or accumulation of GAGs can contribute to developmental disorders, inflammatory conditions, and in some cases, cancer progression.

In retrovirology, Gag refers to the group-specific antigen gene that encodes the Gag polyprotein, which is expressed from the viral genome, directs virion assembly, and is subsequently cleaved by protease into mature structural components such as matrix, capsid, and nucleocapsid.

GAG thus commonly denotes glycosaminoglycans, but may also refer to the retroviral Gag polyprotein in appropriate contexts.