Submitted to: American Society of Plant Biologists
Publication Type: Abstract Only
Publication Acceptance Date: 5/2/2002
Publication Date: N/A
Technical Abstract: Two sucrose synthase isoforms have been identified by activity stained isoelectric focused polyacrylamide electrophoresis in developing sugarbeet (Beta vulgaris L.) root. Sucrose synthase isoform I (SuSyI) was present from the early stages of development to maturity. Sucrose synthase isoform II (SuSyII) was evident only in the late stages of development as roots approached and achieved maturity. The two isoforms were partially purified by combination of ammonium sulfate fractionation, affinity chromatography and anion exchange chromatography. Characterization of the two isoforms revealed that SuSyI and SuSyII share similar kinetic properties and exhibit similar changes ia activity in reposnse to changes in temperature. The two isoforms differed, however, in subunit composition. SuSyI was composed of two 84 kDa subunits and two 86 kDa subunits. SuSyII was composed of four 86 kDa subunits. The two isoforms also differed in their response to pH conditions. SuSyI exhibited maximum sucrose cleaving activity at pH 6.0; SuSyII exhibited maximum sucrose cleaving activity at pH 6.5. The optimum pH for sucrose synthesizing activity occurred at pH 7.5 and pH 7.0 for SuSyI and SuSyII, respectively. At physiological pH values, the two isoforms differed substantially in their response to changes in pH in both the sucrose cleavage and sucrose synthesis reactions. The observed differences suggest that sucrose synthase activity in sugarbeet root may be regulated by differential regulation of expression of the two isoforms and modulation of their activity by changes in cellular pH.