a: Ribbon presentation of the structure of YGR205wp. The central -sheet of the mononucleotide binding fold is shown in yellow, -helices are in blue, the P-loop is in red, the extra -sheet is in green and the lid is in Indian-red. The two bound sulphate ions are shown as ball-and-stick models (yellow for sulphur and red for oxygen). b: Stereo ribbon diagram of the structural superposition of YGR205wp (blue) and pantothenate kinase (red), the structural analogue with the highest Dali Z-score. The sulphate ions belong to the YGR205wp structure and are represented as in part a

The protein product of the YGR205w gene of Saccharomyces cerevisiae was targeted as part of our yeast structural genomics project. YGR205w codes for a small (290 amino acids) protein with unknown structure and function. The only recognizable sequence feature is the presence of a Walker A motif (P loop) indicating a possible nucleotide binding/converting function. We determined the three-dimensional crystal structure of Se-methionine substituted protein using multiple anomalous diffraction. The structure revealed a well known mononucleotide fold and strong resemblance to the structure of small metabolite phosphorylating enzymes such as pantothenate and phosphoribulo kinase. Biochemical experiments show that YGR205w binds specifically ATP and, less tightly, ADP. The structure also revealed the presence of two bound sulphate ions, occupying opposite niches in a canyon that corresponds to the active site of the protein. One sulphate is bound to the P-loop in a position that corresponds to the position of -phosphate in mononucleotide protein ATP complex, suggesting the protein is indeed a kinase. The nature of the phosphate accepting substrate remains to be determined.