Tobacco NtPUB4 is a U-box/Arm protein discovered by its binding to a receptor-like kinase, CHRK1 (Kim et al. 2003). Tobacco seedlings with re duced levels of CHRK1 have widespread developmental defects relating to cell proliferation, such as elevated cytokinin levels, callus formation, shoot overproduction, reduced apical dominance and abnormal flower development (Lee et al. 2003).
Like CHRK1, NtPUB4 localises to the plasma membrane. NtPUB4 is expressed in all tissues, but most highly in flowers (Kim et al. 2003). Loss-of-function of NtPUB4 has not been characterised, so its in vivo function is not clear. However, CHRK1 and NtPUB4 potentially represent players in a novel developmental signalling pathway, as plants with reduced CHRK1 function respond normally to exogenous cytokinins and auxin (Lee et al. 2003). Discovering ligands and interactors will be an exciting area of future research.
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