4-hydroxy-4-methyl-2-oxoglutarate aldolase; part of the gene cluster that mediates the biosynthesis of the tetramic acids Sch210971 and Sch210972, potential anti-HIV fungal natural product that contain a decalin core (PubMed:25885659). The PKS module of tasS together with the enoylreductase tasC catalyze the formation of the polyketide unit which is then conjugated to 4-hydroxyl-4-methyl glutamate (HMG) by the condensation domain of the tasS NRPS module (PubMed:25885659). One unique structural feature of Sch210971 and Sch210972 is the tetramic acid motif proposed to be derived from the non-proteinogenic amino acid HMG, by a Dieckmann-type condensation catalyzed by the reductase domain of tasS (PubMed:25885659). The aldolase tasA catalyzes the aldol condensation of 2 molecules of pyruvic acid to yield the intermediate 4-hydroxyl-4-methyl-2-oxoglutarate (HMOG), which can then be stereoselectively transaminated, may be by tasG, to form HMG (PubMed:25885659). The Diels-Alderase tas3 then uses the Dieckmann product of tasS as substrate and catalyzes the Diels-Alder cycloaddition to form the decalin ring of Sch210971 and Sch210972 (PubMed:25885659). 4-hydroxy-4-methyl-2-oxoglutarate = 2 pyruvate Binds 1 divalent metal cation per subunit. Secondary metabolite biosynthesis. Homohexamer; trimer of dimers. Belongs to the HpcH/HpaI aldolase family.