Discovery of a New Family of Protein Kinases Containing Phosphoinositide 3/4-Kinase and Ubiquitin-Like Domains

Abstract

This thesis is focused on the characterization of new protein kinases from plants. Here, I show that the Arabidopsis UbDK[gamma]4 and UbDK[gamma]7 (ubiquitin-like domain kinases), which were originally identified as putative phosphatidylinositol 4-kinases, have protein kinase activity. As the proposed name UbDK suggests these protein kinases have ubiquitin-like domains in addition to the catalytic phosphoinositide 3/4-kinase domain. In depth characterization of UbDK[gamma]4 revealed its interacting proteins and substrate. UbDKγ4 interacts directly with proteins involved in the ubiquitin⁄proteasome system. Ubiquitin fusion degradation 1 (UFD1), one of UbDK[gamma]4 interacting partners, was identified as a UbDK[gamma]4 substrate and the UFD1 phosphorylation sites were mapped. My thesis is that UbDK[gamma]4 and UbDK[gamma]7 are founding members of a new family of protein kinases.

Analysis of protein-protein interactions, gene expression and preliminary characterization of transgenic plants expressing UbDKγ4-derived polypeptides suggested that UbDKγ4 may be involved in protein degradation during the unfolded protein response in plants. Specifically, we propose that UbDKγ4 is implicated in the recognition and delivery of ubiquitinated proteins derived from the endoplasmic reticulum-associated degradation (ERAD) pathway for degradation via the 26S proteasome.