Proteoglycans of the extracellular environment: clues from the gene and protein side offer novel perspectives in molecular diversity and function.

This review focuses on the extracellular proteoglycans. Special emphasis is placed on the structural features of their protein cores, their gene organization, and their transcriptional control. A simplified nomenclature comprising two broad groups of extracellular proteoglycans is offered: the small leucine-rich proteoglycans or SLRPs, pronounced "slurps, " and the modular proteoglycans. The first group encompasses at least five distinct members of a gene family characterized by a central domain composed of leucine-rich repeats flanked by two cysteine-rich regions. The second group consists of those proteoglycans whose unifying feature is the assembly of various protein modules in a relatively elongated and often highly glycosylated structure. This group is quite heterogeneous and includes a distinct family of proteoglycans, the "hyalectans," that bind hyaluronan and contain a C-type lectin motif that is likely to bind carbohydrates, and a less distinct group that contains structural homologies but lacks hyaluronan-binding properties or lectin-like domains.