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Function

The Sucrose Synthase is able to catalyse the following reaction in both directions:

This reaction is a nucleophilic substitution involving a glycosyl intermediate. The glucose is transfered between UDP (donor) and fructose (acceptor).

This enzyme has a fundamental role in the sucrose flow regulation since it is able to produce sucrose (used for transportation and stockage) and both ADP-glucose and UDP-glucose. It is one of the only four proteins able to synthesize or cleave the sucrose, and the only one able to catalyze it in both direction ^[1], thereby Sucrose synthase is implied in many pathways including cell wall, starch and glycoprotein synthesis ^[2].

Sucrose synthase catalyses the cleavage reaction at pH around 7 and the synthesis reaction at pH above 9. Cytoplasmic pH is around 6,5-7, it suggests that SUS works in vivo only to cleave sucrose, which explain why SUS1 is mostly in sink organs. Therefore, it is a linker between all the pathways that use or synthesize sucrose. SUS is involved in biomass production, nitrogen fixation, fruit and seed maturation and can response to stress (cold, anaerobic conditions, drought or gene induction). However, the binding to target mechanism is unknown at the molecular level.

In a mutant Sus1 (SUS1 gene repression), we observe a decrease in amidon and lipids concentration, a misbalance between hexose and sucrose, and a perturbation of the organic acids synthesis (citrate and malate), during seed maturation

^[3].

Structural highlights

Sucrose synthase is normally a homotetrameric enzyme, but it can also exist in a dimer form ^[4]. There are 6 SUS isoforms in Arabidopsis thaliana and all of them are structurally similar to sucrose phosphate synthases and glycogen synthases. ^[5]. Each monomer is a chain of 808 residues which possesses four specific domains:

• 1-127: N-Terminal regulatory domain involved in targeting (Cellular Targeting Domain) ^[6]. On this sequence, two serines can be phosphorylated, which enable a control of enzyme location ^[7].

• 157-276: EPBD: ENOD40 peptide-binding domain. This domain has a role in the regulation of the enzyme. It is able to bind a potassium ion.

• 277-776 : GT-B glycosyltransferase domain. It contains the catalytic site and presents a characteristic Rossman-folding

^[8]. It is divided in two parts, the GT-BN and the GT-BC.

• 776-808 : C-terminal extension. The length of this domain is variable depending of the SUS isoform.

Primary structure of Atsus1 monomer. CTD : for Cellular Targeting Domain (residues 0-127); L for Linker (residues 128-156); EPBD: for ENOD40 Peptide-Binding Domain (residues 157-276); GT-B glycosyltransferase : Gt-BN (residues 277-526) and Gt-BC (residues 527-754); CTE for C-Terminal Extension (residues 776-808)

The SUS1 tetramer is flat, with two types of hydrophobic subunit interfaces, the A:B and A:D interfaces. The A:D interface is an interaction between the C-terminal extension and the linker, whereas the A:B interface is created by the interaction of adjacent EPBD domains.

The active site of SUS1 is able to bind both fructose and UDP-glucose. UDP-glucose is mainly bound by the GT-BC domain, whereas the fructose in β-furanose form is bound within a pocket in the GT-BN domain.

The GT-B domain is highly conserved in other isoforms and in the Sucrose-Phosphate Synthase. This conservation reinforce the evolutionary relationship of those enzymes. Furthermore, this domain is also conserved in other species.

Regulation

Sucrose Synthase regulation is not well known, but here are some of the supposed mechanisms.

• ENOD40-A is a small hormon-like peptide able to specifically thiolates the Cys-266 of AtSUS1. It also inhibits the phosphorylation of Ser-167, which is within the A:B interface : in this way, in the native tetramer, Ser-167 is inaccessible to phosphorylation, but would be accessible if the tetramer dissociates into dimers upon breaking of the A:B interface (because this interface is less hydrophobic than the A:D interface). So the dissociation into dimers would then allow its phosphorylation and increase the potential for ubiquitinylation and thereby turnover.

• In Arabidopsis, SUS1 gene is probably regulated both by sucrose and D-glucose.

• Conformational changes of the EPBD may change the active site activity through distortions of one of the α helix.

• The CTD can possibly move as a rigid body and interact with F-actin, where the groove and EPBD can be in contact with actin fiber. It may create a pathway for actin binding, and modulation of the Sucrose Synthase activity.

• In other organisms, it has been shown that the Sucrose Synthase is active into its dimer form, it is supposed to be the same phenomenon in Arabidopsis thaliana, though it has not been prouved.