COP1 E3 ligase protects HYL1 to retain microRNA biogenesis

Abstract

Constitutive photomorphogenic 1 (COP1) is a RING-finger E3 ligase that plays a central role in photomorphogenesis by destabilizing many light-regulated transcription factors and photoreceptors. Here, we reveal a novel function for COP1 E3 ligase in controlling global miRNA biogenesis in Arabidopsis thaliana. In cop1 mutants, the level of miRNAs is dramatically reduced because of the diminution of HYPONASTIC LEAVES 1 (HYL1), an RNA-binding protein required for precise miRNA processing. HYL1 is destabilized by an unidentified protease, which we tentatively call protease X, that specifically cleaves the N-terminal region from HYL1, thus neutralizing its function. Our results further show that the cytoplasmic partitioning of COP1 under light is essential to protect HYL1 against protease X. Taken together, we suggest a novel regulatory network involving HYL1, protease X, COP1 and light signalling that is indispensable for miRNA biogenesis in Arabidopsis thaliana.

The activities of the E3 ubiquitin ligase COP1/SPA, a key repressor in light signaling

“…Further experiments propose that the cytosolic fraction of COP1 which is present in light-grown seedlings protects HYL1 from cleavage by an unknown protease. Consistent with this idea full-length HYL1 levels are higher in light-grown than in dark-grown seedlings [75].…”

The SnRK2 kinases modulate miRNA accumulation in Arabidopsis

“…In Arabidopsis , the dephosphorylation of HYL1 by CPL1 is required for accurate miRNA processing and strand selection from miRNA duplexes [20]. A well-studied regulator of photomorphogenesis, constitutive photomorphogenic 1 (COP1), was found to affect miRNA accumulation through regulation of HYL1 [52]. In this study, we showed that SnRK2 kinases affect miRNA expression, possibly by modulating key components of the Arabidopsis miRNA processing complex.…”

“…2/3/6 triple mutant or mannitol-treated seedlings of the snrk2 decuple mutant is due to increased degradation or decreased protein synthesis of HYL1. Recently, Cho et al also found that the level of HYL1 is highly diminished in a cop1 mutant background and suggested involvement of yet to be identified protease X in the proteolysis of HYL1 [52]. It would be interesting to know if there is a potential biochemical link between SnRK2-mediated HYL1 stabilization and COP1-mediated HYL1 stabilization.…”

“…A mass spectrometry experiment of purified HYL1 protein only identified the C-terminal repetitive peptide EAAFGSVETEK as being phosphorylated [20]. It is quite possible that HYL1 is a short-lived protein and very unstable [52], and only the six tandem repeats of this peptide could be detected. It will be very interesting to investigate if these phosphorylation sites are phosphorylated by the SnRK2 kinases in vivo.…”

STV1, a ribosomal protein, binds primary microRNA transcripts to promote their interaction with the processing complex in Arabidopsis

“…For example, HYL1 activity is controlled by phosphorylation and dephosphorylation (30,31). Furthermore, the expression levels of the DCL1 complex are regulated at transcriptional and posttranscriptional levels to influence miRNA accumulation (32)(33)(34). The structure and length of the stem-loop also impact pri-miRNA processing.…”