Petreikov, M., Department of Vegetable Research, Volcani Center-ARO, Bet Dagan 50250, Israel
Eisenstein, M., Chemical Research Support Unit, Weizmann Institute of Science, Rehovot 76100, Israel
Yeselson, Y., Department of Vegetable Research, Volcani Center-ARO, Bet Dagan 50250, Israel
Preiss, J., Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI 48823, United States
Schaffer, A.A., Department of Vegetable Research, Volcani Center-ARO, Bet Dagan 50250, Israel

The enzyme AGPase [ADP-Glc (glucose) pyrophosphorylase] catalyses a rate-limiting step in starch synthesis in tomato (Solanum lycopersicon) fruit, which undergoes a transient period of starch accumulation. It has been a generally accepted paradigm in starch metabolism that the enzyme naturally functions primarily as a heterotetramer comprised of two large subunits (L) and two small subunits (S). The tomato genome harbours a single gene encoding S and three genes for L proteins, which are expressed in both a tissue- and time-specific manner. In the present study the allosteric contributions of the different L subunits were compared by expressing each one in Escherichia coli, in conjunction with S and individually, and characterizing the resulting enzyme activity. Our results indicate different kinetic characteristics of the tomato L1/S and L3/S heterotetramers. Surprisingly, the recombinant L3 protein was also active when expressed alone and size-exclusion and immunoblotting showed that it functioned as a monomer. Subunit interaction modelling pointed to two amino acids potentially affecting subunit interactions. However, directed mutations did not have an impact on subunit tetramerization. These results indicate a hitherto unknown active role for the L subunit in the synthesis of ADP-Glc. © The Authors.