The extended family of collagens

The characteristic triple-helical structure of collagens is not exclusive to the collagen family; it also appears in a wide variety of other proteins where it plays an important functional role. Here, we present an overview of these collagen-like proteins, including their domain structures and functions. We group these collagen-like proteins into seven categories based on function, including the classical complement pathway activators, cytokines, elastin-interfacers, lectins, scavenger receptors, and a miscellaneous category. Unlike actual collagens, which serve important structural roles, the roles of the collagen-like proteins are more diverse; they instead serve a role in cell signaling and in inflammation and immune activation. Examples of important collagen-like proteins include adiponectin, a cytokine regulating metabolism and insulin sensitivity, C1q, a protein involved in activating the innate immune system, and the macrophage receptor with collagen structure that activates macrophages in response to lung infection.