Quinolinate phosphoribosyltransferase (QAPRTase, EC 22.214.171.124) catalyzes the formation of nicotinate mononucleotide, carbon dioxide, and pyrophosphate from 5-phosphoribosyl 1-pyrophosphate (PRPP) and quinolinic acid (QA, pyridine 2,3-dicarboxylic acid). The 2.0 A resolution crystal structure of apo-QAPRTase is a two-domain enzyme, which forms a dimer with the buried surface area of 4970 A2 in the crystal. In each chain, smaller bridging alpha/beta N-terminal domain and bigger TIM-barrel C-terminal domain are interconnected by a ~50 A long alpha helix. The C-terminal half of this helix represents the first helix in the TIM-barrel structure. The N-terminal domain resembles the Rossmann fold and covers the active site of the TIM-barrel part of the V. cholerae apo-QAPRTase.