The Journal of cell biology
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Acute promyelocytic leukemia (APL) is driven by a chromosomal translocation whose product, the PML/retinoic acid (RA) receptor α (RARA) fusion protein, affects both nuclear receptor signaling and PML body assembly. Dissection of APL pathogenesis has led to the rediscovery of PML bodies and revealed their role in cell senescence, disease pathogenesis, and responsiveness to treatment. APL is remarkable because of the fortuitous identification of two clinically effective therapies, RA and arsenic, both of which degrade PML/RARA oncoprotein and, together, cure APL. Analysis of arsenic-induced PML or PML/RARA degradation has implicated oxidative stress in the biogenesis of nuclear bodies and SUMO in their degradation.
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Little is known about quality control of proteins that aberrantly or persistently engage the endoplasmic reticulum (ER)-localized translocon en route to membrane localization or the secretory pathway. Hrd1 and Doa10, the primary ubiquitin ligases that function in ER-associated degradation (ERAD) in yeast, target distinct subsets of misfolded or otherwise abnormal proteins based primarily on degradation signal (degron) location. We report the surprising observation that fusing Deg1, a cytoplasmic degron normally recognized by Doa10, to the Sec62 membrane protein rendered the protein a Hrd1 substrate. ⋯ Mutations that prevent translocon engagement caused a reversion to Doa10-dependent degradation. Similarly, a variant of apolipoprotein B, a protein known to be cotranslocationally targeted for proteasomal degradation, was also a Hrd1 substrate. Hrd1 therefore likely plays a general role in targeting proteins that persistently associate with and potentially obstruct the translocon.
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MicroRNAs (miRNAs) are noncoding RNAs that function as key posttranscriptional regulators of gene expression. miRNA maturation is controlled by the DROSHA microprocessor complex. However, the detailed mechanism of miRNA biogenesis remains unclear. We show that the tumor suppressor breast cancer 1 (BRCA1) accelerates the processing of miRNA primary transcripts. ⋯ We also found that BRCA1 recognizes the RNA secondary structure and directly binds with primary transcripts of miRNAs via a DNA-binding domain. Together, these results suggest that BRCA1 regulates miRNA biogenesis via the DROSHA microprocessor complex and Smad3/p53/DHX9. Our findings also indicate novel functions of BRCA1 in miRNA biogenesis, which may be linked to its tumor suppressor mechanism and maintenance of genomic stability.
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Suppression of autophagy is always accompanied by marked accumulation of p62, a selective autophagy substrate. Because p62 interacts with the Nrf2-binding site on Keap1, which is a Cullin 3-based ubiquitin ligase adapter protein, autophagy deficiency causes competitive inhibition of the Nrf2-Keap1 interaction, resulting in stabilization of Nrf2 followed by transcriptional activation of Nrf2 target genes. ⋯ Gene targeting of p62 in an HCC cell line markedly abrogates the anchorage-independent growth, whereas forced expression of p62, but not a Keap1 interaction-defective mutant, resulted in recovery of the growth defect. These results indicate the involvement of persistent activation of Nrf2 through the accumulation of p62 in hepatoma development.
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Biography Historical Article
Ramanujan Hegde: The prion puzzle and protein translocation. Interview by Caitlin Sedwick.