Crystallizing ideas about Parkinson's disease.

G enetic forms of Parkinson’s disease (PD) are useful in helping us to understand the underlying pathophysiology of this disorder in both its rare familial forms and the more common ‘‘sporadic’’ type. Three genes are unambiguously causal in different PD families: -synuclein, parkin, and DJ-1 (reviewed in ref. 1). Two recessive mutations have been found in DJ-1, a large deletion and an L166P point mutation (2). DJ-1 mutations are likely to result in loss of protein function, and, hence, deciphering the normal cellular role of DJ-1 will be key to understanding how mutations cause disease. DJ-1 has a number of functions, several of which may be relevant to the pathways that underlie PD (3). It was cloned independently in different laboratories examining processes as varied as cellular transformation, RNA binding, and male fertility (4–6). For this reason, and because DJ1 is a member of a large superfamily of proteins, we have to be cautious in assigning a PD-specific function. In this issue of PNAS, the report by Wilson et al. (7) of the detailed crystal structure of DJ-1 adds considerably to our understanding of what DJ-1 is and what it is not. Two additional studies showing DJ-1 at different resolutions are now in press (8, 9). All three structures show a similar overall folding pattern and demonstrate that DJ-1 is a dimer, which is confirmed in two of these papers by additional techniques. There are similarities to previously solved structures within the DJ-1 superfamily, namely the bacterial proteins PH1704 (10), a protease, and heat shock protein 31 (Hsp31) (7), a chaperone from Escherichia coli. Although the different proteins in the superfamily have a general pattern of folding that is similar (Fig. 1), there are also substantial differences. For example, PH1704 is a hexamer compared with the dimeric nature of DJ-1 and Hsp31. In each of the recently identified structures of DJ-1, there are key residues that are highlighted in relation to the known or purported function of the protein. Given the relevance to human disease, locating the site of mutations is an important contribution. Leucine 166 is in the penultimate C-terminal -helix, and the L166P mutation is strongly predicted to disrupt this structural motif. Our recent results suggest that L166P destabilizes the protein (D. W. Miller, R. Ahmad, S. Hague, M. J. Baptista, R. Canet-Aviles, C. McLendon, D. M. Carter, P.-P. Zhu, J. Stadler, J. Chandran, G. R. Klinefelter, C. Blackstone, and M.R.C., unpublished data), in line with the prediction that helixes G and H together contribute to dimer formation. This is also consistent with predictions previously made by Bonifati et al. (2), who based their structural model on the structural homologue PH1704. Solving the crystal structure of DJ-1 allows us to test predictions about the function of the protein, or more accurately to suggest that some homologybased ideas are unlikely to be correct. In other members of the DJ-1 superfamily, a catalytic Glu/Asp–His–Cys triad is responsible for enzyme activity. Although this triad is present in DJ-1, it appears not to be catalytically competent as the arrangement of the conserved residues is not suitable for proton transfer, as occurs in the nucleophilic attack mechanism used in proteases, amidotransferases, or kinase members of the DJ-1 superfamily. In fact, two of the structural studies report negative results in protease and kinase assays (7, 9), although one paper does label a potential active site (8). Interestingly, a His–Cys pair is oriented in a similar manner to DJ-1 in some groups of proteases such as the caspases (12) that have been shown to act as cellular executioners. Amidotransferase activity is unlikely, because there is no suitable glutamine-binding site. Overall, these results suggest that DJ-1 is not likely to be an enzyme in any of the known classes represented by other members of the DJ-1 superfamily, but do not yet exclude the possibility that there is some form of catalysis. The equivalent residue to the catalytically active cysteine in human DJ-1 is Cys106, and structural data indicate that this is a reactive amino acid. Given the differ-