Reversible post-translational modifications of numerous proteins by small ubiquitinrelated modifiers (SUMO) play a major role in regulating a broad array of basic cellular and developmental processes. Genetic studies have shown, for example, that SUMOylation is essential for early embryogenesis in animals and plants. While SUMO target proteins are predominantly conjugated to monomeric SUMO, polySUMOylation has also been shown to occur. In grasses we have now identified the first protein, DSUL, naturally consisting of two head-to-tail SUMO-like protein domains, similar to mammalian Role and targets of diSUMO-like DSUL FAT10 and ISG15 that contain two ubiquitin-like (UbL) domains. Expression of DSUL is restricted to the micropylar region of the female gametophyte and zygote. Functional studies indicated that DSUL is required for nuclei positioning, cell specification and viability during female gametogenesis in maize. We will now study whether (i) DSUL uses the SUMO enzymatic machinery for conjugation and deconjugation or an own machinery for DSULylation and deDSULylation, identify (ii) DSUL substrates in maize as a prerequisite to study the mechanisms through which DSUL modification of proteins regulates cellular and developmental processes. DSUL target proteins shall (iii) be compared to SUMO und Ub targets in maize. Finally, we will (iv) elucidate the biological role of DSUL for asymmetric zygote division and spindle organization.
