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Muscle actin and fructose-1,6-bisphosphate aldolase (aldolase) were chemically crosslinked to produce an 80 kDa product representing one subunit of aldolase linked to one subunit of actin. Hydroxylamine digestion of the crosslinked product resulted in two 40.5 kDa fragments, one that was aldolase linked to the 12 N-terminal residues of actin. Brownian dynamics simulations of muscle aldolase and GAPDH with F-actin (muscle, yeast, and various mutants) estimated the association free energy. Mutations of residues 1-4 of muscle actin to Ala individually or two in combination of the first four residues reduced the estimated binding free energy. Simulations showed that muscle aldolase binds with the same affinity to the yeast actin as to the double mutated muscle actin; these mutations make the N-terminal of muscle actin identical to yeast, supporting the conclusion that the actin N-terminus participates in binding. Because the depth of free energy wells for yeast and the double mutants is less than for native rabbit actin, the simulations support experimental findings that muscle aldolase and GAPDH have a higher affinity for muscle actin than for yeast actin. Furthermore, Brownian dynamics revealed that the lower affinity of yeast actin for aldolase and GAPDH compared to muscle actin, was directly related to the acidic residues at the N-terminus of actin.

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