Background: The Acel_2062 protein from Acidothermus cellulolyticus is a protein of unknown function. Initial\r\nsequence analysis predicted that it was a metallopeptidase from the presence of a motif conserved amongst the\r\nAsp-zincins, which are peptidases that contain a single, catalytic zinc ion ligated by the histidines and aspartic acid\r\nwithin the motif (HEXXHXXGXXD). The Acel_2062 protein was chosen by the Joint Center for Structural Genomics\r\nfor crystal structure determination to explore novel protein sequence space and structure-based function annotation.\r\nResults: The crystal structure confirmed that the Acel_2062 protein consisted of a single, zincin-like\r\nmetallopeptidase-like domain. The Met-turn, a structural feature thought to be important for a Met-zincin because\r\nit stabilizes the active site, is absent, and its stabilizing role may have been conferred to the C-terminal Tyr113. In our\r\ncrystallographic model there are two molecules in the asymmetric unit and from size-exclusion chromatography, the\r\nprotein dimerizes in solution. A water molecule is present in the putative zinc-binding site in one monomer, which is\r\nreplaced by one of two observed conformations of His95 in the other.\r\nConclusions: The Acel_2062 protein is structurally related to the zincins. It contains the minimum structural features of\r\na member of this protein superfamily, and can be described as a ââ?¬Å?mini- zincinââ?¬Â. There is a striking parallel with the\r\nstructure of a mini-Glu-zincin, which represents the minimum structure of a Glu-zincin (a metallopeptidase in which\r\nthe third zinc ligand is a glutamic acid). Rather than being an ancestral state, phylogenetic analysis suggests that the\r\nmini-zincins are derived from larger proteins.
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