Collagenous Transmembrane Proteins: Recent Insights into Biology and Pathology*

Is collagen just the workhorse of the extracellular matrix with few roles other than merely giving the body structural platforms? Hardly so. The collagen family has now swelled to at least 27 distinct members and thus is full of promise for functional diversity (1). Granted, some collagens belong to the best known and most abundant structural proteins. However, collagens also control sophisticated organ or tissue functions by means which are unexpected, fascinating, and often bewildering. Like other matrix macromolecules, collagens are functional only after aggregating into tissue suprastructures.