ADP-glucose pyrophosphorylase (AGPase) catalyzes the first committed step of starch synthesis in plants. The potato tuber enzyme contains a pair of catalytic small subunits (SSs) and a pair of non-catalytic large subunits (LSs). We have previously identified a LS mutant containing a P52L replacement, which rendered the enzyme with down-regulatory properties. To investigate the structure-function relationships between the two subunits with regard to allosteric regulation, putative SS mutants that could reverse the down-regulatory condition of LSP52L were identified by their ability to restore glycogen accumulation in an AGPase-deficient Escherichia coli glgC-strain. Two distinct LS-dependent classes, bona fide SS suppressors dependent on LSP52L but not LSWT and SS up-regulating allosteric mutants, were evident by kinetic analysis. These results indicate that both LS and SS have a regulatory function in controlling allosteric properties through enhancing cooperative subunit interactions. (C) 2007 Elsevier Inc. All rights reserved.