Department of Biochemistry, Saga Medical School, Nabeshima, Saga, Japan. kusakabe@bcmp.med.harvard.edu
Three isoforms of fructose-1,6-bisphosphate aldolase were found to bind specifically to the actin-containing filament of the cytoskeleton and to show tissue-specific binding patterns. Aldolase A (muscle type) bound more tightly to the skeletal muscle cytoskeleton among the three isozymes, while aldolase B (liver type) preferred the liver cytoskeleton to those of other tissues. The specific binding of aldolase A to the skeletal muscle cytoskeleton was inhibited strongly by the substrates fructose 1,6-bisphosphate and fructose 1-phosphate. Several mutant aldolases A were examined to identify the amino acid residues or regions that play a role in specific binding. Among the mutant aldolases tested, A-E34D, A-K41N, and A-Y363S exhibited remarkably reduced binding activities. Experiments using FITC-labeled enzymes and Rh-labeled phalloidin disclosed that aldolase A associated with the cytoskeleton. Specifically, when aldolase A was incubated with human fibroblast MRC-5 permeabilized with Triton X-100, aldolase A bound to the actin filaments in the stress fibers within the cell. Aldolase A reversibly inhibited the contraction of MRC-5 cells which usually occurred in the presence of Mg2(+)-ATP and Ca2+. These results provide direct evidence that aldolase binds specifically to the actin-containing stress fibers and suggest that aldolase may regulate cell contraction through its reversible binding to the filaments in the permeabilized MRC-5 fibroblast.