Fructose-1,6-bisphosphatase from Synechococcus leopoliensis: substrate-dependent dimer―tetramer interconversion

Extracts of Synechococcus leopoliensis (Anacystis nidulans) contain two forms of D-fructose-1,6-bisphosphatase (EC 3.1.3.11) previously designated as forms A and B [Gerbling, K.-P., Steup, M., and Latzko, E. (1984) Arch. Microbiol. 137, 109-114]. Form B, which probably represents the major part of the total extractable fructose-1,6-bisphosphatase activity, has been purified to apparent homogeneity. Gel filtration, non-denaturing polyacrylamide gel electrophoresis, and cross-linking with bis(sulfosuccinimidyl)suberate revealed that the fructose-1,6-bisphosphatase B exists in either a dimeric or in a tetrameric subform, depending upon the absence or presence of fructose-1,6-bisphosphate and Mg2+. The dimer--tetramer interconversion was readily reversible. The results provide evidence for a two-step activation of fructose-1,6-bisphosphatase B involving the reduction of the dimeric subform and the subsequent substrate-dependent conversion of the reduced dimer to a reduced tetramer, which is the only catalytically active state. In contrast to form B, no substrate-dependent interconversion was detected with form A from S. leopoliensis.