proteoglycan

If collagen fibers are the cables that resist pulling, something else must resist squeezing — like the water-filled gel inside a memory-foam cushion that pushes back when you press it. The extracellular matrix needs exactly this kind of springy, water-holding filler. Proteoglycans are that filler: matrix molecules built to soak up water and form a swollen gel that resists compression and cushions tissues.

A proteoglycan is a core protein with many long sugar chains attached to it, sticking out like the bristles of a bottle brush. These sugar chains (called glycosaminoglycans) carry a heavy load of negative electric charge. Negative charges repel each other and also attract positively charged ions, which in turn pull in water, so a proteoglycan draws a large shell of water around itself and swells into a firm, hydrated gel. Many proteoglycans can link onto a backbone molecule to form enormous complexes that fill the spaces between collagen fibers, creating a matrix that is strong in tension (from collagen) and resistant to squeezing (from the proteoglycan gel).

Proteoglycans matter wherever tissues must bear pressure and bounce back. Cartilage in your joints is packed with them, which is why it can cushion the pounding of every step and why it springs back after being compressed. They also help control what diffuses through the matrix and can grip signaling molecules, holding them in place for cells to read. A subtle but important point: their cushioning comes from trapped water, not from the molecules being intrinsically hard — squeeze cartilage slowly and water seeps out, which is part of why joints stiffen when they are overloaded or as cartilage wears with age.