A dual mechanism controls nuclear localization in the atypical basic-helix-loop-helix protein PAR1 of Arabidopsis thaliana.

PAR1 is an atypical basic-helix-loop-helix (bHLH) protein that negatively regulates the shade avoidance syndrome in Arabidopsis thaliana acting as a transcriptional cofactor. Consistently with this function, PAR1 has to be in the nucleus to display biological activity. Previous structure-function analyses revealed that the N-terminal region of PAR1 drives the protein to the nucleus. However, truncated forms of PAR1 lacking this region still display biological activity, implying that PAR1 has additional mechanisms to localize into the nucleus. In this work, we compared the primary structure of PAR1 and various related and unrelated plant bHLH proteins, which led us to suggest that PAR1 contains a non-canonical nuclear localization signal (NLS) in the N-terminal region. By overexpressing truncated and mutated derivatives of PAR1, we have also investigated the importance of other regions of PAR1, such as the acidic and the extended HLH dimerization domains, for its nuclear localization. We found that, in the absence of the N-terminal region, a functional HLH domain is required for nuclear localization. Our results suggest the existence of a dual mechanism for PAR1 nuclear localization: (1) one mediated by the N-terminal non-consensus NLS and (2) a second one that involves interaction with other proteins via the dimerization domain.