Protein-protein interactions are essential for many cellular processes. We have developed a technology called light-activated dimerization (LAD) to artificially induce protein hetero- and homodimerization in live cells using light. Using the FKF1 and GIGANTEA (GI) proteins of Arabidopsis thaliana, we have generated protein tags whose interaction is controlled by blue light. We demonstrated the utility of this system with LAD constructs that can recruit the small G-protein Rac1 to the plasma membrane and induce the local formation of lamellipodia in response to focal illumination. We also generated a light-activated transcription factor by fusing domains of GI and FKF1 to the DNA binding domain of Gal4 and the transactivation domain of VP16, respectively, showing that this technology is easily adapted to other systems. These studies set the stage for the development of light-regulated signaling molecules for controlling receptor activation, synapse formation and other signaling events in organisms.

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Caspases are cysteine proteases that are key effectors in apoptotic cell death. Currently, there is a lack of tools that can be used to monitor the regulation of specific caspases in the context of distinct apoptotic programs. We describe the development of highly selective inhibitors and active site probes and their applications to directly monitor executioner (caspase-3 and -7) and initiator (caspase-8 and -9) caspase activity. Specifically, these reagents were used to dissect the kinetics of caspase activation upon stimulation of apoptosis in cell-free extracts and intact cells. These studies identified a full-length caspase-7 intermediate that becomes catalytically activated early in the pathway and whose further processing is mediated by mature executioner caspases rather than initiator caspases. This form also shows distinct inhibitor sensitivity compared to processed caspase-7. Our data suggest that caspase-7 activation proceeds through a previously uncharacterized intermediate that is formed without cleavage of the intact zymogen.

SUMMARYVacuolar processing enzymes (VPEs) are important cysteine proteases that are implicated in the maturation of seed storage proteins, and programmed cell death during plant-microbe interactions and development. Here, we introduce a specific, cell-permeable, activity-based probe for VPEs. This probe is highly specific for all four Arabidopsis VPEs, and labeling is activity-dependent, as illustrated by sensitivity for inhibitors, pH and reducing agents. We show that the probe can be used for in vivo imaging and displays multiple active isoforms of VPEs in various tissues and in both monocot and dicot plant species. Thus, VPE activity profiling is a robust, simple and powerful tool for plant research for a wide range of applications. Using VPE activity profiling, we discovered that VPE activity is increased during infection with the oomycete pathogen Hyaloperonospora arabidopsidis (Hpa). The enhanced VPE activity is host-derived and EDS1-independent. Sporulation of Hpa is reduced on vpe mutant plants, demonstrating a role for VPE during compatible interactions that is presumably independent of programmed cell death. Our data indicate that, as an obligate biotroph, Hpa takes advantage of increased VPE activity in the host, e.g. to mediate protein turnover and nutrient release.

We introduce a new method for purifying recombinant proteins expressed in bacteria using a highly specific, inducible, self-cleaving protease tag. This tag is comprised of the Vibrio cholerae MARTX toxin cysteine protease domain (CPD), an autoprocessing enzyme that cleaves exclusively after a leucine residue within the target protein-CPD junction. Importantly, V. cholerae CPD is specifically activated by inositol hexakisphosphate (InsP6), a eukaryotic-specific small molecule that is absent from the bacterial cytosol. As a result, when His6-tagged CPD is fused to the C-terminus of target proteins and expressed in Escherichia coli, the full-length fusion protein can be purified from bacterial lysates using metal ion affinity chromatography. Subsequent addition of InsP6 to the immobilized fusion protein induces CPD-mediated cleavage at the target protein-CPD junction, releasing untagged target protein into the supernatant. This method condenses affinity chromatography and fusion tag cleavage into a single step, obviating the need for exogenous protease addition to remove the fusion tag(s) and increasing the efficiency of tag separation. Furthermore, in addition to being timesaving, versatile, and inexpensive, our results indicate that the CPD purification system can enhance the expression, integrity, and solubility of intractable proteins from diverse organisms.

STIM2β is a novel STIM2 splice isoform that inhibits Orai channels.

The papain-family cathepsins are cysteine proteases that are emerging as promising therapeutic targets for a number of human disease conditions ranging from osteoporosis to cancer. Relatively few selective inhibitors for this family exist, and the in vivo selectivity of most existing compounds is unclear. We present here the synthesis of focused libraries of epoxysuccinyl-based inhibitors and their screening in crude tissue extracts. We identified a number of potent inhibitors that display selectivity for endogenous cathepsin targets both in vitro and in vivo. Importantly, the selectivity patterns observed in crude extracts were generally retained in vivo, as assessed by active-site labeling of tissues from treated animals. Overall, this study identifies several important compound classes and highlights the use of activity-based probes to assess pharmacodynamic properties of small-molecule inhibitors in vivo.

Store-operated calcium (SOC) channels are vital for activation of the immune cells, and mutations in the channel result in severe combined immunodeficiency in human patients. In lymphocytes, SOC entry is mediated by the Orai1 channel, which is activated by direct binding of STIM1. Here we describe an alternative approach for identifying inhibitors of SOC entry using minimal functional domains of STIM1 and Orai1 to screen a small-molecule microarray. This screen identified AnCoA4, which inhibits SOC entry at submicromolar concentrations and blocks T cell activation in vitro and in vivo. Biophysical studies revealed that AnCoA4 binds to the C terminus of Orai1, directly inhibiting calcium influx through the channel and also reducing binding of STIM1. AnCoA4, unlike other reported SOC inhibitors, is a molecule with a known binding site and mechanism of action. These studies also provide proof of principle for an approach to ion channel drug discovery.