Post-translational modification by transglutaminase of proteins involved in incompatibility

Self-incompatibility has been investigated in the cv Abbè Fétel (Pyrus communis) in order to detect a role for 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. In the Abbè Fétel styles pollinated with Williams pollen (A x W, compatible system), on the contrary, the activity decreased during pollen germination. The TGase gene has been cloned and sequenced from Abbè 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 only the activity was stimulated by some factor dependent on SI. A molecular approach to clone one of the S-RNase alleles of the cv Abbè Fétel has been performed. This protein, supplied to pollen in the germination medium, was able to inhibit the growth of the 50% of tubes of the homologous pollen. This approach will allow 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.