Structural Insights into Triglyceride Storage Mediated by Fat Storage-Inducing Transmembrane (FIT) Protein 2

Fat storage-Inducing Transmembrane proteins 1 & 2 (FIT1/FITM1 and FIT2/FITM2) belong to a unique family of evolutionarily conserved proteins localized to the endoplasmic reticulum that are involved in triglyceride lipid droplet formation. FIT proteins have been shown to mediate the partitioning of cellular triglyceride into lipid droplets, but not triglyceride biosynthesis. FIT proteins do not share primary sequence homology with known proteins and no structural information is available to inform on the mechanism by which FIT proteins function. Here, we present the experimentally-solved topological models for FIT1 and FIT2 using N-glycosylation site mapping and indirect immunofluorescence techniques. These methods indicate that both proteins have six-transmembrane-domains with both N- and C-termini localized to the cytosol. Utilizing this model for structure-function analysis, we identified and characterized a gain-of-function mutant of FIT2 (FLL(157-9)AAA) in transmembrane domain 4 that markedly augmented the total number and mean size of lipid droplets. Using limited-trypsin proteolysis we determined that the FLL(157-9)AAA mutant has enhanced trypsin cleavage at K86 relative to wild-type FIT2, indicating a conformational change. Taken together, these studies indicate that FIT2 is a 6 transmembrane domain-containing protein whose conformation likely regulates its activity in mediating lipid droplet formation

How should we approach classification of autoinflammatory diseases?

International audienceThe notion of 'autoinflammatory' disease was introduced at the end of the 1990s, and, since then, this concept has rapidly evolved. As a result, multiple definitions of autoinflammatory disease, and classifications of conditions encompassed by these definitions, have been proposed; this succession highlights advances that have been made in understanding of the innate immune system, and especially the roles of IL-1β and the inflammasome in autoinflammtory conditions. However, the definitions and classifications that have been suggested to date face a number of structure and content issues. We therefore propose another, more clinically-oriented, definition: autoinflammatory diseases are diseases with clinical signs of inflammation, associated with elevated levels of acute-phase reactants, which are attributable to dysfunction of the innate immune system, genetically-determined or triggered by an endogenous factor. From this foundation, we propose a clinically-based classification of autoinflammatory diseases, and go on to discuss how immunological diseases as a whole, including autoimmune diseases, can be appropriately located within a continuum only if the classification process is multidimensional. For this purpose, we appeal to the philosophical concepts of family resemblance and signature