The ability of rabbit liver aldolase (D-fructose-1,6-bisphosphate D-glyceraldehyde-3-phosphatate-lyase, EC 4.1.2.13) and rabbit liver fructose-1,6-bisphosphatase (FruP2ase; D-fructose-1,6-bisphosphate 1-phosphohydrolase, EC 3.1.3.11) to partition into the gel phase of Ultrogel AcA 34 is decreased in a mixture of the two enzymes. Titration experiments indicate that a 1:1 complex is formed. The value for the distribution coefficient of the complex corresponds to a molecular mass of 300,000 daltons, the value expected for a dimer containing one mole of each enzyme protein. Complex formation was not observed when either liver enzyme was replaced by the corresponding isozyme from rabbit muscle. The susceptibility of liver Fru-Pease to limited proteolysis by subtilisin was reduced in the presence of liver aldolase, but not when the latter was replaced by muscle aldolase, suggesting that the conformation of Fru-P2ase is altered in the complex. Limited proteolysis of liver aldolase abolishes its ability both to form theterodimer and to protect Fru-P2ase from modification by subtilisin.In mammalian liver fructose-1,6-bisphosphate aldolase (D-fructose-1,6-bisphosphate D-glyceraldehyde-3-phosphate-lyase, EC 4.1.2.13) and fructose-1,6-bisphosphatase fructose-1,6-bisphosphate 1-phosphohydrolase, EC 3.1.3.11) catalyze successive reactions in gluconeogenesis. Both are found in the cytosol and, like other enzymes of glycolysis and gluconeogenesis, are considered to exist and function as independent entities. In recent years, however, evidence has been accumulating for loose interactions between cytosolic proteins and between these proteins and cellular matrices. Arnold et al. (1,2) have reported that several glycolytic enzymes of muscle, including aldolase and glyceraldehyde-3-phosphate dehydrogenase, tend to associate with muscle actin, and Clarke and Masters (3) have examined these interactions in detail under conditions of physiological ionic strength. In mammalian erythrocytes both aldolase (4, 5) and glyceraldehyde-3-phosphate dehydrogenase (6) bind to the major membrane polypeptide (band 3 polypeptide), suggesting that these enzymes may function as a membrane-bound complex. Kinetic and physicochemical evidence for an interaction between aldolase and glyceraldehyde-3-phosphate dehydrogenase purified from rabbit muscle has been reported by Keleti and his coworkers (7,8).It has been proposed (3, 9, 10) that weak interactions between soluble enzymes may play a role in metabolism, either by promoting substrate channeling, in the case of enzymes that catalyze successive reactions, or by the induction of conformation changes that alter their catalytic or regulatory properThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact. 3889 ties. Although direct evidence for subtle interactions has been difficult to obtain, the kinetic evidence for substrate channeling repo...