Enzyme activity--it's all about image

Unraveling the functional roles of proteins is a major challenge facing the postgenome researcher. Advances towards this goal have been made through the development of both chemical and biochemical tools for monitoring protein activity. Recently, a myriad of fluorescence-based imaging tools have emerged for in vitro, in vivo and whole animal applications. These tools have provided methods to monitor the spatial and temporal distribution of proteins and bioorganic molecules dynamically. Here, recent advances in chemical and biochemical techniques that allow the detection of enzymatic activity within intact cells and in vivo are reviewed. Such technologies have the potential to be integrated into drug-development programs to facilitate both the functional validation of pharmaceutical targets and the treatment of human disease.     

ALS mutants of human superoxide dismutase form fibrous aggregates via framework destabilization

Many point mutations in human Cu,Zn superoxide dismutase (SOD) cause familial amyotrophic lateral sclerosis (FALS), a fatal neurodegenerative disorder in heterozygotes. Here we show that these mutations cluster in protein regions influencing architectural integrity. Furthermore, crystal structures of SOD wild-type and FALS mutant H43R proteins uncover resulting local framework defects. Characterizations of beta-barrel (H43R) and dimer interface (A4V) FALS mutants reveal reduced stability and drastically increased aggregation propensity. Moreover, electron and atomic force microscopy indicate that these defects promote the formation of filamentous aggregates. The filaments resemble those seen in neurons of FALS patients and bind both Congo red and thioflavin T, suggesting the presence of amyloid-like, stacked beta-sheet interactions. These results support free-cysteine-independent aggregation of FALS mutant SOD as an integral part of FALS pathology. They furthermore provide a molecular basis for the single FALS disease phenotype resulting from mutations of diverse side-chains throughout the protein: many FALS mutations reduce structural integrity, lowering the energy barrier for fibrous aggregation.     

Murid herpesvirus 4 strain 68 M2 protein is a B-cell-associated antigen important for latency but not lymphocytosis

This work describes analyses of the function of the murid herpesvirus 4 strain 68 (MHV-68) M2 gene. A frameshift mutation was made in the M2 open reading frame that caused premature termination of translation of M2 after amino acid residue 90. The M2 mutant showed no defect in productive replication in vitro or in lungs after infection of mice. Likewise, the characteristic transient increase in spleen cell number, Vbeta4 T-cell-receptor-positive CD8(+) T-cell mononucleosis, and establishment of latency were unaffected. However, the M2 mutant virus was defective in its ability to cause the transient sharp rise in latently infected cells normally seen in the spleen after infection of mice. We also demonstrate that expression of M2 is restricted to B cells in the spleen and that M2 encodes a 30-kDa protein localizing predominantly in the cytoplasm and plasma membrane of B cells.     

Amyloid as a natural product

Amyloid fibrils, such as those found in Alzheimer's and the gelsolin amyloid diseases, result from the misassembly of peptides produced by either normal or aberrant intracellular proteolytic processing. A paper in this issue by Marks and colleagues (Berson et al., 2003) demonstrates that intra-melanosome fibrils are formed through normal biological proteolytic processing of an integral membrane protein. The resulting peptide fragment assembles into fibrils promoting the formation of melanin pigment granules. These results, along with the observation that amyloid fibril formation by bacteria is highly orchestrated, suggest that fibril formation is an evolutionary conserved biological pathway used to generate natural product nanostructures.     

A microplate assay specific for the enzyme aggrecanase

We have identified a 41-residue peptide, bracketing the aggrecanase cleavage site of aggrecan, that serves as a specific substrate for this enzyme family. Biotinylation of the peptide allowed its immobilization onto streptavidin-coated plates. Aggrecanase-mediated hydrolysis resulted in an immobilized product that reveals an N-terminal neoepitope, recognized by the specific antibody BC-3. This assay is highly specific for aggrecanases; MMPs were inactive in this assay. Reduction of the peptide size below 30 amino acids resulted in a significant diminution of activity. Using the immobilized 41-residue peptide as a substrate, we have developed a 96-well microplate-based assay that can be conveniently used for high-throughput screening of samples for aggrecanase activity and for discovery of inhibitors of aggrecanase activity.     

Cys10 mixed disulfides make transthyretin more amyloidogenic under mildly acidic conditions

Conservative mutation of transthyretin's surface residues can predispose an individual to familial amyloidosis by dramatically changing the energetics of misfolding. Senile systemic amyloidosis (SSA), however, cannot be explained in this fashion because wild-type (WT) transthyretin (TTR) misfolds and misassembles into amyloid. Since various modifications of the SH functionality of Cys10 have been reported in humans, we sought to understand the extent to which these modifications alter the stability and amyloidosis of WT TTR as a possible explanation for SSA. Homotetrameric Cys10 TTR variants, including TTR-Cys, TTR-GSH, TTR-CysGly, and S-sulfonated TTR, were chemically synthesized starting with WT TTR. The TTR-Cys, TTR-GSH, and TTR-CysGly isoforms are more amyloidogenic than WT at the higher end of the acidic pH range (pH 4.4-5.0), and they are similarly destabilized relative to WT TTR toward urea denaturation. They exhibit rates of urea-mediated tetramer dissociation (pH 7) and MeOH-facilitated fibril formation similar to those of WT TTR. Under mildly acidic conditions (pH 4.8), the amyloidogenesis rates of the mixed disulfide TTR variants are much faster than the WT rate. S-Sulfonated TTR is less amyloidogenic and forms fibrils more slowly than WT under acidic conditions, yet it exhibits a stability and rates of tetramer dissociation similar to those of WT TTR when subjected to urea denaturation. Conversion of the Cys10 SH group to a mixed disulfide with the amino acid Cys, the CysGly peptide, or glutathione increases amyloidogenicity and the amyloidogenesis rate above pH 4.6, conditions under which TTR probably forms fibrils in humans. Hence, these modifications may play an important role in human amyloidosis.     

Amino acid determinants in cyclooxygenase-2 oxygenation of the endocannabinoid anandamide

The endocannabinoid arachidonylethanolamide (AEA, anandamide) is an endogenous ligand for the cannabinoid receptors and has been shown to be oxygenated by cyclooxygenase-2 (COX-2). We examined the structural requirements for COX-mediated, AEA oxygenation using a number of substrate analogues and site-directed mutants of COX-2. Fourteen AEA analogues were synthesized and tested as COX substrates. These studies identified the hydroxyl moiety of AEA as a critical determinant in the ability of COX enzymes to effect robust endocannabinoid oxygenation. In addition, these studies suggest that subtle structural modifications of AEA analogues near the ethanolamide moiety can result in pronounced changes in their ability to serve as COX-2 substrates. Site-directed mutagenesis studies have permitted the development of a model of AEA binding within the COX-2 active site. As with arachidonic acid, the omega-terminus of AEA binds in a hydrophobic alcove near the top of the COX-2 active site. The polar ethanolamide moiety of AEA, like the carboxylate of arachidonate, interacts with Arg-120 at the bottom of the COX-2 active site. Mutation of Tyr-385 prevents AEA oxygenation, suggesting that, as in the case of other COX substrates, AEA metabolism is initiated by Tyr-385-mediated hydrogen abstraction. Thus, AEA binds within the COX-2 active site in a conformation roughly similar to that of arachidonic acid. However, important differences have been identified that account for the isoform selectivity of AEA oxygenation. Importantly, the COX-2 side pocket and Arg-513 in particular are critical determinants of the ability of COX-2 to efficiently generate prostaglandin H(2) ethanolamide. The reduced efficiency of COX-1-mediated, AEA oxygenation can thus be explained by the absence of an arginine residue at position 513 in this isoform. Mutational analysis of Leu-531, an amino acid located directly across from the COX-2 side pocket, suggests that AEA is shifted away from this hydrophobic residue and toward Arg-513 relative to arachidonic acid. Coupled with earlier observations with the endocannabinoid 2-arachidonylglycerol, these results indicate that one possible function of the highly conserved COX-2 active site side pocket is to promote endocannabinoid oxygenation.     

Polar targeting of DivIVA in Bacillus subtilis is not directly dependent on FtsZ or PBP 2B

DivIVA is involved in Bacillus subtilis cell division and is located at the cell poles. Previous experiments suggested that the cell division proteins FtsZ and PBP 2B are required for polar targeting of DivIVA. By using outgrowing spores, we show that DivIVA accumulates at the cell poles independent of the presence of FtsZ or PBP 2B.