Post-Translational Modification by Transglutaminase of Proteins Involved in Pear Self-Incompatibility.

Self-incompatibility has been investigated in the cultivar ‘Abbé Fétel’ (Pyrus communis) in order to assign a role to transglutaminase (TGase), an enzyme able to post-translationally forming cross-links among proteins. This enzyme is localised in the pollen cytoplasm where it regulates the polymerisation of cytoskeleton proteins. An extracellular form is also necessary for pollen tube growth. In the self-pollinated style of Abbè Fétel (A x A, incompatible system), the activity of TGase increased when the pollen tube stopped its growth inside the style and the enzyme was immuno-localised around the tube tip. On the contrary in Abbé Fétel styles pollinated with Williams pollen (A x W, compatible system), TGase activity decreased during pollen germination. The TGase gene has been cloned and sequenced from Abbé Fétel pollen and style showing that it shares a high homology with a sequence of apple, whose genome has been recently published, which presents the typical TGase catalytic sequence, and with the TGase of Arabidopsis. This enzyme was expressed during pollen germination inside the style to a similar extent in both systems (A x A and A x W) showing that it is only the activity to have been stimulated by some factor dependent on SI. A molecular approach to clone one of the S-RNase alleles of the cv Abbé Fétel has been performed. This protein, supplied to pollen in the germination medium, was able to inhibit the growth of 50% of the tubes of the homologous pollen. This approach will allow us to verify if an interaction between the two enzymes will occur during SI, as TGase, at least in animal cells, is also able to act as disulphide isomerase modifying the RNase structure.