Rice sucrose-phosphate synthase: Identification of an isoform specific for heterotrophic tissues with distinct metabolite regulation from the mature leaf enzyme

Abstract

Immunohistological analyses for rice (Oryza sati7a) sucrose- to Pi which also strongly decreased enzyme activation by phosphate synthase (SPS, UDP-glucose D-fructose-6-phos- Glc-6-P. SPS-2 was highly activated by Glc-6-P and phate-2-glucosyltransferase, EC 2.4.1.14) show that the showed low sensitivity to Pi. In vitro alkaline phosphatase protein is differently localized in photosynthetic and etio- treatment suggested that SPS-1 could be regulated as leaf lated leaves. Very little is known about SPS regulation in SPS in darkness and that SPS-2 is present in a dephosphoheterotrophic tissues; therefore, we studied the biochemical rylated state or is not regulated by protein phosphorylation. properties of the enzyme from etiolated seedlings and em- The relative MM value (116 kDa) estimated for both SPS bryo. Two SPS forms (SPS-1 and SPS-2) were partially forms in SDS-PAGE is identical to the rice leaf SPS purified from etiolated seedlings. The effects of Glc-6-P (ac- polypeptide. Taken together, these data led us to conclude tivator) and Pi (inhibitor) on SPS activities allowed us to that SPS-2 is an enzyme form only present in non-photodifferentiate the two forms. SPS-1 showed high sensitivity synthetic tissues.