UPL5 modulates WHY2 protein distribution in a Kub-site dependent ubiquitination in response to [Ca2+]cyt-induced leaf senescence.

The translocation of proteins between various compartments of cells is the simplest and most direct way of an/retrograde communication. However, the mechanism of protein trafficking is far understood. In this study, we showed that the alteration of WHY2 protein abundance in various compartments of cells was dependent on a HECT-type ubiquitin E3 ligase UPL5 interacting with WHY2 in the cytoplasm, plastid, and nucleus, as well as mitochondrion to selectively ubiquitinate various Kub-sites (Kub 45 and Kub 227) of WHY2. Plastid genome stability can be maintained by the UPL5-WHY2 module, accompany by the alteration of photosystem activity and senescence-associated gene expression. In addition, the specificity of UPL5 ubiquitinating various Kub-sites of WHY2 was responded to cold or CaCl2 stress, in a dose [Ca2+]cyt-dependent manner. This demonstrates the integration of the UPL5 ubiquitination with the regulation of WHY2 distribution and retrograde communication between organelle and nuclear events of leaf senescence.