ADP-glucose pyrophosphorylase (AGP) represents a key regulatory step in polysaccharide synthesis in organisms ranging from bacteria to plants. Higher plant AGPs are complex in nature and are heterotetramers consisting of two similar but distinct subunits. How the subunits are assembled into enzymatically active polymers is not yet understood. Here, we address this issue by using naturally occurring null mutants of the Shrunken2 (Sh2) and Brittle2 (Bt2) loci of maize as well as the yeast two-hybrid expression system. In the absence of the maize endosperm large AGP subunit (SH2), the BT2 subunit remains as a monomer in the developing endosperm. In contrast, the SH2 protein, in the absence of BT2, is found in a complex of 100 kD. A direct interaction between SH2 and BT2 was proven when they were both expressed in yeast. Several motifs are essential for SH2:BT2 interaction because truncations removing the N or C terminus of either subunit eliminate SH2:BT2 interactions. Analysis of subunit interaction mutants (sim) also identified motifs essential for protein interactions.