Pectin methyltransferase

In most plant materials that have been examined, several PME isoforms have been extracted fixDm the cell walls. What is their reqjective fimction How is their synthesis regulated -Immunolocalisation experiments have shown that some Golgi vesicles can be labelled with the JIM 5 antibody, which is known to recognize low-ester pectins [19]. This raises the question of the action pattern of the pectin methyltransferases. Can they produce pectins with different... 

These acidic molecules might result from either the lack of PMT activity or the action of PME, known to be present in most plant cell walls. Pectin methyltransferases and pectin methylesterases extracted from active and resting cells were therefore characterized. 

Pectin methyltransferases from active and resting cells of flax. 

The possibility that the initial degree of methyl-esterification might be controlled by the properties of the methyltransferase enzymes was examined partial characterisation of these enzymes in suspension-cultured cells of fiax. Pectin methyltransferases beii enzymes characteristic of the Golgi apparatus [22], microsomes were fiactionated daily for ten days from suspension-cultured flax cells and incubated in the presence of C-SAM, the universal donor of methyl groups. 

With endogeneous pectic polysaccharides as substrates, the pectin methyhransferase activity was measured as radioactivity linked to oxalate-soluble polys x harides after extensive washing of microsomes with IM ethanolic NaCL Figure 2 shows that the rate of methylesterification of pectic substances was maximal on days 4 and 6 these maximum activities were observed within this period in at least five independent ejqjeriments. On the other hand, little activity was noted in young cells before day 2, and in old cells after day 9. In other words during the stationary phase the newly synthesised pectins remained unesterified because of the lack of pectin methyltransferase activity. 

Figure 2 Pectin methyltransferase activity of supension-cultured cells of flax.

In these experiments the pectin methyltransferase activity in vitro was generally only a few picokatal g" of protein, due mainly to the limited amount of SAM and endogeneous polysaccharide substrate. 

PECTIN METHYLTRANSFERASES FROM SUSPENSION-CULTURED CELLS AND SEEDLINGS OF FLAX Linunt mitatissimum L. ). 

Figure 1 indicates that pectin methyltransferase (PMT) activity from freeze-thawed microsomes measured without exogenous substrate was maximal at neutral pH (6.5 to 7.5). When exogenous pectic substrates of various DE had been added, similar optimal neutral pH was observed, and the activity was slightly stimulated (1.2 to 1.8 times). A second optimal pH occured at pH 5.5, but in the presence of low methylated pectin (DE 0.1). As suggested by Lineweaver and Ballou [8] to explain the behaviour of another pectic enzyme -i.e. pectin methylesterase (PME), the mobility and the activity of PMT might be influenced by the presence of polyanionic substrates. On the other hand, the existence of several forms of pectin methyltransferase in flax microsomes might be responsible for such variations of the activity. 

Bruyant-Vannier et al [5] have shown that pectin methyltransferases solubilized from endomembranes of flax cells consisted of several polypeptides, the molecular relative mass of which varied from very low (Mr < 5,000) to very high (Mr ca 200,000) values. 

Figure 1. Effect of pH and of the addition of exogeneous pectins of low and high degree of esterification on the pectin methyltransferase activity fi oin fi eze-thawed microsomes of flax cells.

Table 1 Pectin methyltransferase activity in dpm per organ of flax seedlings...

Figure 2 Specific activity of pectin methyltransferases fi om microsomes of flax seedlings. A cotyledons B hypocotyls C roots. The activity was measured at pH 5.5 ( ) and 7 (o)...

GOUBET, F., COUNCIL, L. N., MOHNEN, D., Identification and partial characterization of the pectin methyltransferase "homogalacturonan-methyltransferase from membranes of tobacco cell suspensions., Plant Physiol., 1998,116,337-347. 

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