Root phototropism protein 3 (RPT3; also known as nonphototropic hypocotyl 3, NPH3) and root phototropism 2 (RPT2) (IPR029958) represent the founding members of a novel plant-specific family [1]. Three domains define the members of this family: an N-terminal BTB (broad complex, tramtrack, bric a brac) domain (IPR000210), a centrally located NPH3 domain (IPR027356), and a C-terminal coiled-coil domain.

NPH3 assembles with CUL3 to form a E3 complex that ubiquitinates phototropin 1 (phot1) and modulates phototropic responsiveness [2, 3]. NPH3 is necessary for root and hypocotyl phototropisms, but not for the regulation of stomata opening or chloroplast relocation [4]. Coleoptile phototropism protein 1 (CPT1) is a rice orthologue of Arabidopsis NPH3 also required for phototropism [5]. This entry also includes DOT3 (AT5G10250) that is involved in shoot and primary root growth; dot3 mutants produce an aberrant parallel venation pattern in juvenile leaves [6].

1. Understanding phototropism: from Darwin to today. J. Exp. Bot. 60, 1969-78
2. Cullins 3a and 3b assemble with members of the broad complex/tramtrack/bric-a-brac (BTB) protein family to form essential ubiquitin-protein ligases (E3s) in Arabidopsis. J. Biol. Chem. 280, 18810-21
3. Modulation of phototropic responsiveness in Arabidopsis through ubiquitination of phototropin 1 by the CUL3-Ring E3 ubiquitin ligase CRL3(NPH3). Plant Cell 23, 3627-40
4. RPT2 is a signal transducer involved in phototropic response and stomatal opening by association with phototropin 1 in Arabidopsis thaliana. Plant Cell 16, 887-96
5. The Rice COLEOPTILE PHOTOTROPISM1 gene encoding an ortholog of Arabidopsis NPH3 is required for phototropism of coleoptiles and lateral translocation of auxin. Plant Cell 17, 103-15
6. Vein patterning screens and the defectively organized tributaries mutants in Arabidopsis thaliana. Plant J. 56, 251-63

Root phototropism protein 3 (RPT3; also known as nonphototropic hypocotyl 3, NPH3) and root photo
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