Methods for site-specific modification of proteins are in high demand. Reactions that yield bioconjugates should be quantitative, site-specific, and versatile with respect to nature and size of the biological/chemical targets involved, require minimal modification of the target, display acceptable kinetics under physiological conditions, and be orthogonal to other labeling methods. Sortase-mediated transpeptidation reactions meet these criteria. Here we describe the expression and purification conditions for two orthogonal sortase A enzymes and provide the protocol that allows functionalization of any given protein at its C-terminus or for select proteins at an internal site. Sortase-mediated reactions take only a few minutes, but reaction times can be extended to increase yields.

Deficiency of glucocerebrosidase (GBA) underlies Gaucher disease, a common lysosomal storage disorder. Carriership for Gaucher disease has recently been identified as major risk for parkinsonism. Presently, no method exists to visualize active GBA molecules in situ. We here report the design, synthesis and application of two fluorescent activity-based probes allowing highly specific labeling of active GBA molecules in vitro and in cultured cells and mice in vivo. Detection of in vitro labeled recombinant GBA on slab gels after electrophoresis is in the low attomolar range. Using cell or tissue lysates, we obtained exclusive labeling of GBA molecules. We present evidence from fluorescence-activated cell sorting analysis, fluorescence microscopy and pulse-chase experiments of highly efficient labeling of GBA molecules in intact cells as well as tissues of mice. In addition, we illustrate the use of the fluorescent probes to study inhibitors and tentative chaperones in living cells.

The proteasome is an essential evolutionary conserved protease involved in many regulatory systems. Here, we describe the synthesis and characterization of the activity-based, fluorescent, and cell-permeable inhibitor Bodipy TMR-Ahx(3)L(3)VS (MV151), which specifically targets all active subunits of the proteasome and immunoproteasome in living cells, allowing for rapid and sensitive in-gel detection. The inhibition profile of a panel of commonly used proteasome inhibitors could be readily determined by MV151 labeling. Administration of MV151 to mice allowed for in vivo labeling of proteasomes, which correlated with inhibition of proteasomal degradation in the affected tissues. This probe can be used for many applications ranging from clinical profiling of proteasome activity, to biochemical analysis of subunit specificity of inhibitors, and to cell biological analysis of the proteasome function and dynamics in living cells.

For many proteins, the N- or the C-terminus make essential contributions to substrate binding, for protein-protein interactions, or for anchoring the proteins to a membrane. In other circumstances, at least one of the termini is buried within the protein, rendering it inaccessible to labeling. The possibility of selective modification of one of the protein’s termini may present unique opportunities for biochemical and biological applications. We describe sortase-mediated reactions to selectively label the N-terminus of a protein with a variety of functional groups. If sortase, the protein of interest, and a suitably functionalized label are available, the reactions usually require less than 3 hours.