Butyrophilins (BTN) belong to the immunoglobulin (Ig) superfamily of transmembrane proteins. These molecules are of increasing interest to immunologists, as they share a structural homology with B7 family members at the extracellular domain level. Moreover, a role of these molecules has been suggested in the negative regulation of lymphocyte activation for almost all the BTN that have been studied. In addition, the expression of some BTN family members has been reported to be associated with autoimmune diseases. Over the last few years, the number of BTN and BTN-like members has greatly increased. In this study, the butyrophilin family in mammals has been revisited, using phylogenetic analysis to identify all the family members and the phylogenetic relations among them, and to establish a standard nomenclature. Fourteen BTN groups were identified that are not all conserved between mammalian species. In addition, an overview of expression profiles and functional BTN data demonstrates that these molecules represent a new area of investigation for the design of future strategies in the modulation of the immune system.

Tight regulation of collagen fibril deposition in the extracellular matrix is essential for normal tissue homeostasis and repair, defects in which are associated with several degenerative or fibrotic disorders. A key regulatory step in collagen fibril assembly is the C-terminal proteolytic processing of soluble procollagen precursors. This step, carried out mainly by bone morphogenetic protein-1/tolloid-like proteinases, is itself subject to regulation by procollagen C-proteinase enhancer proteins (PCPEs) which can dramatically increase bone morphogenetic protein-1/tolloid-like proteinase activity, in a substrate-specific manner. Although it is known that this enhancing activity requires binding of PCPE to the procollagen C-propeptide trimer, identification of the precise binding site has so far remained elusive. Here, use of small-angle X-ray scattering provides structural data on this protein complex indicating that PCPE binds to the stalk region of the procollagen C-propeptide trimer, where the three polypeptide chains associate together, at the junction with the base region. This is supported by site-directed mutagenesis, which identifies two highly conserved, surface-exposed lysine residues in this region of the trimer that are essential for binding, thus revealing structural parallels with the interactions of Complement C1r/C1s, Uegf, BMP-1 (CUB) domain-containing proteins in diverse biological systems such as complement activation, receptor signaling, and transport. Together with detailed kinetics and interaction analysis, these results provide insights into the mechanism of action of PCPEs and suggest clear strategies for the development of novel antifibrotic therapies.proteolysis | structural proteins | fibrosis R egulation of collagen deposition in the extracellular matrix is essential for normal tissue homeostasis and repair, defects in which are associated with numerous, often lethal, disorders, including chronic wounds and the many different forms of fibrosis (affecting heart, lung, liver, kidney, skin, etc.) (1, 2). At the protein level, a key regulatory step is the proteolytic conversion of soluble precursor molecules, procollagens, by removal of N-and C-propeptides (3), resulting in spontaneous assembly of collagen fibrils. Although collagen deposition in vivo is also controlled by numerous interactions with cell-surface and extracellular matrix proteins (4, 5), it is generally acknowledged that a key ratelimiting step is the proteolytic removal of the procollagen Cpropeptides. The principal proteinases involved here are zinc metalloproteinases, called bone morphogenetic protein-1 (BMP-1)/tolloid-like proteinases (collectively known as BTPs), which cleave the C-propeptides from the major fibrillar procollagens (types I, II, and III) (3,(6)(7)(8).Although BTPs cleave a number of extracellular substrates (including structural proproteins, proenzymes, and latent growth factors or their antagonists) (8, 9), their activity on fibrillar procollagens is specifically increased, several-fold, by procollag...