Blocking the activity of IL-1␤ has entered the clinical arena of treating autoimmune diseases. However, a successful outcome of this approach requires a clear definition of the mechanisms controlling IL-1␤ release. These are still unclear as IL-1␤, lacking a secretory signal peptide, follows a nonclassical pathway of secretion. Here, we analyze the molecular mechanism(s) undergoing IL-1␤ processing and release in human monocytes and provide a unifying model for the regulated secretion of the cytokine. Our data show that in a first step, pro-caspase-1 and endotoxininduced pro-IL-1␤ are targeted in part to specialized secretory lysosomes, where they colocalize with other lysosomal proteins.

The possibility of predicting the overall shape of a macromolecule in solution from its diffusional properties has gained increasing importance in the structural genomic era. Here we explore and quantify the influence that unstructured and flexible regions have on the motions of a globular protein, a situation that can occur from the presence of such regions in the natural sequence or from additional tags. I27, an immunoglobulin-like module from the muscle protein titin, whose structure and properties are well characterized, was selected for our studies. The backbone dynamics and the overall tumbling of three different constructs of I27 were investigated using (15)N NMR relaxation collected at two (15)N frequencies (60.8 and 81.1 MHz) and fluorescence depolarization spectroscopy after labeling of a reactive cysteine with an extrinsic fluorophore. Our data show that the presence of disordered tags clearly exerts a frictional drag that increases with the length of the tags, thus affecting the module tumbling in solution. We discuss the use and the limitations of current approaches to hydrodynamic calculations, especially when having to take into account local flexibility.