Beyond degradation: COP1 fine-tunes plant growth through non-destructive protein tagging

In the dark (e.g., under soil), hypocotyls elongate, but when exposed to light, they stop elongation. The biosynthesis of the plant hormone auxin is implicated in the elongation of hypocotyls in the dark. In the light, however, auxin action is inhibited, thereby limiting hypocotyl elongation. The E3 ubiquitin ligase CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) promotes elongated hypocotyl in the dark, but is inactivated in light. Ubiquitins are small, evolutionarily conserved (“ubiquitous”) proteins that, through the action of ubiquitin ligases like COP1, are attached to target proteins. When the link is made at lysine 48 (K48) of ubiquitin, the tagged proteins are targeted for degradation in the proteasome. In this article, Liu et al., showed that COP1 is not all about protein degradation as seen in the more common K48 linkage, but can also play a non-proteolytic role via linkage at lysine 63 (K63). The authors experimentally showed that to promote hypocotyl elongation in the dark, COP1 adds a specific molecular tag (K63-linked ubiquitin) to the enzyme GRETCHEN HAGEN 3.5 (GH3.5). This tagging inhibits the activity of GH3.5, thereby limiting the conjugation of amino acids to auxin (indole-3-acetic acid: IAA), suppressing hypocotyl elongation. However, in the light, COP1 inactivation disrupts K63 ubiquitination of GH3.5, leading to the activation of GH3.5, leading to increased amino acid conjugation to IAA, thus limiting IAA levels that promote hypocotyl elongation. Here, the authors showed a mechanism by which light regulates auxin metabolism to modulate hypocotyl elongation. (Summary by Nathaniel Oragbon @NathanIgwe) Nature Comms. 10.1038/s41467-025-58767-6