Ficolins and the fibrinogen-like domain

Ficolins are a group of proteins containing collagen-hire and fibrinogen-like (FBG) sequences and they have a similar overall structure to Clq and the collectins. There are two types of ficolin in man: L-ficolin and M-ficolin. L-ficolin is synthesized in the liver and secreted into the plasma. It binds to several apparently unrelated structures including sugar residues and enhances phagocytosis of bound bacteria. M-ficolin is synthesized mainly in monocytes and is detected on the monocyte surface. The polypeptide sequences of ficolins, the collectins and Clq diverge mainly in their C-terminal globular regions which are, respectively, FBG domains, Ca2+-dependent: carbohydrate recognition domains (C-type CRD), and collagen-related sequences. The FBG domain consists of 220-250 residues and is found in a number of proteins besides fibrinogen and ficolins. The crystal structure of the FBG domain has been characterized and the elucidation of its binding properties should provide essential insights into its role in ficolins and other proteins.