27.7:
Matrix Proteoglycans and Glycoproteins
Matrix proteoglycans are composed of several glycosaminoglycans chains covalently attached to a core protein. They are commonly found in the extracellular matrix or ECM and at the cell membrane.
In contrast, in glycoproteins, a protein core is covalently attached to oligosaccharide chains. They come in different forms and are usually found on the cell membrane, in the ECM, and in blood.
Cells communicate with other cells by secreting signaling proteins that travel through the dense ECM network to reach their designated cells.
The proteoglycans with multiple binding sites act as local reservoirs for vascular endothelial growth factors that are responsible for cell proliferation.
In this way, proteoglycans limit the diffusion of the growth factor, localizing their effect on nearby cells.
Proteoglycans also protect several signaling proteins from degradation by proteases.
Similarly, glycoproteins, such as integrins, serve as cell surface receptors for another glycoprotein called fibronectin. These two glycoproteins together play major roles in connecting cells with the ECM.
27.7:
Matrix Proteoglycans and Glycoproteins
Proteoglycans are extensively glycosylated proteins, commonly found in the extracellular matrix, interwoven with collagen fibers. Hyaline cartilage, the most common type of cartilage in the body, consists of short and dispersed collagen fibers associated with large amounts of proteoglycans. These proteoglycans have long negative charges that attract cations, which in turn attract water molecules. This influx of ions and water molecules swells up the proteoglycan like a water-soaked gel that can withstand the compression forces.
The two common examples of matrix glycoproteins are integrin, laminin, and fibronectin. Integral proteins, or integrins, as their name suggests, integrate completely into the membrane structure with their hydrophobic membrane-spanning regions interacting with the phospholipid bilayer's hydrophobic region. Integral proteins may serve as enzymes, as structural attachments for the cytoskeleton's fibers, or as part of the cell’s recognition sites. Collagen and proteoglycan are attached to fibronectin proteins, which are further attached to integrin proteins. For example, the bacterium Streptococcus pyogenes, which is known to cause strep throat and various other infections, attach to fibronectin present in the extracellular matrix of the cells lining the oropharynx. Laminins are important for cell migration, growth and differentiation. In association with type IV collagens, the laminins form an interconnected network that provides strength and flexibility to the basal lamina.
This text is adapted from:
Openstax, Anatomy and physiology 2e, Section 4.3: Connective tissue supports and protects.
Openstax, Biology 2e, Section 5.1: Components and Structure.
Openstax, Microbiology, Section 3.4: Unique characteristics of Eukaryotic cells.