Exploring the distinct structural arrangement of subunits in the heterotetrameric rice ADP-glucose pyrophosphorylase

ADP-glucose pyrophosphorylase plays a pivotal role as an allosteric enzyme essential for starch biosynthesis in plants. The higher plant AGPase is composed of a pair of large and a pair of small subunits to form a heterotetrameric complex. Growing evidence indicates that each subunit plays a distinct role in regulating the underlying mechanism of starch biosynthesis. In rice genome, there are four large subunit genes (OsL1-L4) and three small subunit genes (OsS1, OsS2a, and OsS2b). While the structural complex between cytosolic rice AGPase subunits (OsL2:OsS2b) has been resolved, no such interaction study has been thus far reported for plastidial rice AGPase (OsL1:OsS1). In this study, we employed protein modeling and MD simulation approaches to gain insights into the interaction between plastidial AGPase subunits. The results revealed a unique OsL1:OsS1 heterotetramer structure, consisting of two up-side-down homodimers and two side-by-side heterodimers. This structure differs from cytosolic OsL2:OsS2b and potato plastidial AGPase heterotetramers. Moreover, results from yeast-two-hybrid experiment indicated a potential difference in the arrangement of OsL1:OsS1 and OsL2:OsS2b. Hence, plastidial AGPase (OsL1:OsS1) found in rice culm and developing endosperm may exhibit unique regulatory and catalytic mechanisms when compared to its counterpart, OsL2:OsS2b, which is predominantly present in the rice endosperm.