The role of IBR5-AtNRPB4 interaction in Arabidopsis growth and development
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Auxin is a crucial plant hormone necessary for the regulation of growth and development in plants. It has been shown that Indole Butyric acid Response 5 (IBR5), which encodes a dual specificity phosphatase, is involved in the auxin signaling pathway. However, the exact molecular mechanism by which it regulates auxin signaling is not well understood. In yeast two-hybrid screen, IBR5 interacted with NRPB4, which is known to be an integral part of RNA Polymerase II complex. Previous studies on NRPB4 have shown that it is involved in thermotolerance in yeast. In arabidopsis, NRPB4 is seen to physically interacts with IBR5 in-vitro confirming yeast two-hybrid result. Further analysis shows that the catalytic domain of IBR5 is important for the interaction with AtNRPB4. The biological relevance of IBR5-AtNRPB4 interaction is not yet clear, but both proteins have been linked to heat stress responses. Characterization of Arabidopsis nrpb4 mutants show severe growth defects in hypocotyl elongation and root growth in response to high temperature showing defective auxin responses suggesting possible functional interaction between IBR5 and NRPB4 by itself or as a complex with other auxin-related protein such as HSP90. However, effort to demonstrate IBR5-NRPB4 interaction in vivo was not successful. Nevertheless, in-vitro interaction between IBR5- NRPB4 along with common defective phenotypes of nrpb4 and ibr5 mutants indicate a possible interaction between these two proteins.