Sucrose synthase (SUS) is phosphorylated on a major, amino-terminal site located at Ser-15 (S15) in the maize (Zea mays) SUS1 protein. Site-and phospho-specific antibodies against a phosphorylated S15 (pS15) peptide allowed direct analysis of S15 phosphorylation in relation to membrane association. Immunoblots of the maize leaf elongation zone, divided into 4-cm segments, demonstrated that the abundance of soluble (s-SUS) and membrane (m-SUS) SUS protein showed distinct positional profiles. The content of m-SUS was maximal in the 4-to 8-cm segment where it represented 9% of total SUS and occurred as a peripheral membrane protein. In contrast, s-SUS was highest in the 12-to 16-cm segment. Relative to s-SUS, m-SUS was hypophosphorylated at S15 in the basal 4 cm but hyperphosphorylated in apical segments. Differing capabilities of the antipS15 and anti-S15 peptide antibodies to immunoprecipitate SUS suggested that phosphorylation of S15, or exposure of unphosphorylated SUS to slightly acidic pH, altered the structure of the amino terminus. These structural changes were generally coincident with the increased sucrose cleavage activity that occurs at pH values below 7.5. In vitro S15 phosphorylation of the S170A SUS protein by a maize calcium-dependent protein kinase (CDPK) significantly increased sucrose cleavage activity at low pH. Collectively, the results suggest that (1) SUS membrane binding is controlled in vivo; (2) relative pS15 content of m-SUS depends on the developmental state of the organ; and (3) phosphorylation of S15 affects aminoterminal conformation in a way that may stimulate the catalytic activity of SUS and influence membrane association.In maize (Zea mays), the enzyme sucrose synthase (SUS) is usually a homo-or heterotetrameric protein complex (Su and Preiss, 1978;Chourey et al., 1986) composed of subunits encoded by the sh1, sus1, and sus3 genes (Carlson et al., 2002). The SUS protein is expressed in many organs including maize endosperm, roots, stems, and leaves (Chourey et al., 1986;Nguyen-Quoc et al., 1990;Carlson et al., 2002) and catalyzes a readily reversible reaction that interconverts Suc and UDP into UDP-Glc and Fru (Tsai, 1974). The pH optimum for the Suc cleavage activity of SUS occurs at pH 6.0 to 6.5, while the synthesis of Suc by SUS is maximal at alkaline pH (8.0-8.8;Tsai, 1974;Su and Preiss, 1978). Therefore, the Suc cleavage capability of SUS increases substantially as cytosolic pH is lowered over the physiological range of 7.6 to 6.8. Such decreases in intracellular pH are known to occur, for example, during anoxia (Gout et al., 2001). The activity of SUS increases under hypoxic and/or anoxic conditions, and tolerance to these stresses is dependent upon SUS activity (Ricard et al., 1998;Zeng et al., 1998;Subbaiah and Sachs, 2001). SUS is particularly enriched in heterotrophic organs, and the importance of its Suc cleavage activity is evidenced by SUS mutants that show reductions in cellulose, callose, and starch formation Sturm and Tang, 1999;Subbaiah and Sachs, 2001).The solub...