THE 1.8 ANGSTROM CRYSTAL-STRUCTURE OF THE YCAC GENE-PRODUCT FROM ESCHERICHIA-COLI REVEALS AN OCTAMERIC HYDROLASE OF UNKNOWN SPECIFICITY

Background: The ycaC gene comprises a 621 base pair open reading frame in Escherichia coli. The ycaC gene product (ycaCgp) is uncharacterize d and has no assigned function. The closest sequence homologs with an assigned function belong to a family of bacterial hydrolases that cata lyze isochorismatase-like reactions, but these have only low sequence similarity to ycaCgp (similar to 20% amino acid identity). The ycaCgp was obtained and identified during crystallization trials of an unrela ted E. coli protein with which it co-purified. Results: The 1.8 Angstr om crystal structure of ycaCgp reveals an octameric complex comprised of two tetrameric rings. A large three-layer (apa) sandwich domain and a small helical domain form the folded structure of the monomeric uni t. Comparisons with sequence and structure databases suggest that ycaC gp belongs to a diverse family of bacterial hydrolases. The most close ly related three-dimensional structure is that of the D-2 tetrameric N -carbamoylsarcosine amidohydrolase (CSHase) from an Arthrobacter speci es. A conspicuous cleft between two ycaCgp subunits contains several c onserved residues including Cys118, which we propose to be catalytic. In the active site, a nonprolyl cis peptide bond precedes Val114 and c oincides with a cis peptide bond in CSHase in a region of dissimilar s equence. The crystal structure reveals a probable error or mutation re lative to the reported genomic sequence. Conclusions: Although the spe cific function of ycaCgp is not yet known, structural studies solidify the relationship of this protein to other hydrolases and illuminate i ts active site and key elements of the catalytic mechanism.