Like NR, sucrose-phosphate synthase (SPS) [EC 2.4.1.14] is also regulated by reversible phosphorylation (Huber and Huber, 1996).

Sucrose-phosphate phosphatase (SPP) [EC 3.1.3.24] keeps the sucrose-6'-P low, and thereby renders the SPS reaction essentially irreversible (Huber and Huber, 1996). There is evidence that SPS and SPP associate to form a multienzyme complex (Echeverria et al, 1997).

Site-specific seryl phosphorylation of spinach SPS on Ser-158 is the mechanism underlying light/dark modulation of SPS. A calcium-independent kinase catalyzes phosphorylation and inactivation of dephosphoserine-158-SPS. Sugden et al (1999) have characterized calcium-independent kinases in spinach, and have identified two major activities, HRK-A and HRK-C (3-hydroxy-3-methylglutaryl-coenzyme A reductase kinase A and C). These kinases not only phosphorylate 3-hydroxy-3-methylglutaryl-coenzyme A reductase, but also inactivate spinach sucrose phosphate synthase via phosphorylation at Ser-158, and phosphorylate nitrate reductase purified from spinach at Ser-543 (Sugden et al, 1999). These kinases therefore potentially regulate several major biosynthetic pathways in plants: isoprenoid synthesis, sucrose synthesis, and nitrogen assimilation (Sugden et al, 1999).

In contrast to the phosphorylated NR-14-3-3 complex which is activated by dissociation with 14-3-3-binding phosphopeptides, the total sugar-phosphate synthase activity in plant extracts was inhibited by up to 40% by a 14-3-3-binding phosphopeptide and the phosphopeptide-inhibited activity was reactivated by adding excess 14-3-3 proteins (Moorhead et al, 1999).

SPS shows a circadian pattern of activity in tomato. SPS is most active in its dephosphorylated state, which normally coincides with daytime. Jones and Ort (1997) suggest that there is a circadian rhythm controlling the transcription of a protein phosphatase that subsequently dictates the circadian rhythm in SPS activity via effects on this enzyme's phosphorylation state. Chilling stess disrupts this circadian rhythm probably by affecting the expression of the SPS phosphatase (Jones et al, 1998). [In contrast, chilling-induced delay in NR activity does not arise from effects on NR phosphorylation, but direct effects on NR expression; Jones et al, 1998)].

In addition, SPS is activated by osmotic stress in darkened spinach. This involves phosphorylation of Ser-424, catalyzed by a calcium-dependent, 150 kDa protein kinase (Toroser and Huber, 1997).

Echeverria E, Salvucci ME, Gonzalez P, Paris G, Salerno G 1997 Physical and kinetic evidence for an association between sucrose-phosphate synthase and sucrose-phosphate phosphatase. Plant Physiol. 115: 223-227.

Huber SC, Huber JL 1996 Role and regulation of sucrose-phosphate synthase in higher plants. Annu. Rev. Plant Physiol. Plant Mol. Biol. 47: 431-444.

Jones TL, Ort DR 1997 Circadian regulation of sucrose phosphate synthase activity in tomato by protein phosphatase activity. Plant Physiol. 113: 1167-1175.

Jones TL, Tucker DE, Ort DR 1998 Chilling delays circadian pattern of sucrose phosphate synthase and nitrate reductase activity in tomato. Plant Physiol. 118: 149-158.

Moorhead G, Douglas P, Cotelle V, Harthill J, Morrice N, Meek S, Deiting U, Stitt M, Scarabel M, Aitken A, MacKintosh C 1999 Phosphorylation-dependent interactions between enzymes of plant metabolism and 14-3-3 proteins. Plant J. 18: 1-12.

Sugden C, Donaghy PG, Halford NG, Grahame Hardie D 1999 Two SNF1-related protein kinases from spinach leaf phosphorylate and inactivate 3-hydroxy-3-methylglutaryl-coenzyme A reductase, nitrate reductase, and sucrose phosphate synthase in vitro. Plant Physiol. 120: 257-274.

Toroser D, Huber SC 1997 Protein phosphorylation as a mechanism for osmotic stress activation of sucrose-phosphate synthase in spinach leaves. Plant Physiol. 114: 947-955.