Autophagy | The Role of RPS6KA3/RSK2-mediated DRAM2 Phosphorylation in Regulating Lysosomal Targeting and Autophagic Flux in Melanoma

This study reveals that the RPS6KA3-DRAM2 signaling axis acts as a molecular switch regulating autophagy and exosome secretion through a phosphorylation-dependent sorting mechanism, providing a novel mechanistic model and intervention node for experimental design in melanoma and other autophagy-related malignancies.

 

Literature Overview

This article, titled 'RPS6KA3/RSK2-mediated phosphorylation of DRAM2 promotes lysosomal targeting and autophagic flux in melanoma,' published in the journal Autophagy, systematically investigates how the kinase RPS6KA3/RSK2 regulates the transport of the lysosomal membrane protein DRAM2 from late endosomes to lysosomes through phosphorylation, thereby maintaining the integrity of autophagic flux. The study further reveals the pro-tumorigenic role of this pathway in melanoma progression, expanding our understanding of the spatially specific regulatory mechanisms of autophagy.

Background Knowledge

Melanoma is a highly aggressive form of skin cancer, and its treatment resistance and recurrence are often closely associated with tumor cells' ability to adapt to microenvironmental stresses (e.g., nutrient deprivation, hypoxia). Growing evidence indicates that autophagy plays a critical role in sustaining tumor cell survival, especially in advanced disease. However, how to precisely regulate autophagy initiation and completion, particularly the molecular switches governing lysosomal targeting, remains a major bottleneck in current research. Most existing studies focus on ubiquitination or lipidation modifications of classical autophagy substrates such as SQSTM1 and LC3B, while dynamic regulatory mechanisms of lysosomal membrane protein trafficking remain poorly understood. This study focuses on the question: Do specific kinases determine the subcellular localization and functional fate of membrane proteins by phosphorylating their cytoplasmic domains? Based on bioinformatic prediction and IP-MS screening, the authors identified RPS6KA3 as the upstream kinase of DRAM2, and further elucidated the pathological significance of this phosphorylation-dependent sorting mechanism in melanoma. Notably, AP3D1, a core subunit of the adaptor protein complex AP-3, has been reported to participate in the lysosomal sorting of various membrane proteins, but how its recognition signals are dynamically regulated remains unclear. This study provides a new perspective on the reversible regulation of lysosomal trafficking and offers potential intervention strategies for targeting the late stages of autophagic flux.

 

 

Research Methods and Experiments

The authors first used NetPhos and GPS kinase prediction tools to screen for potential phosphorylation sites on DRAM2, and identified RPS6KA3 as an interacting kinase via IP-MS. By constructing phosphorylation-deficient (DRAM2S263A) and phospho-mimetic (DRAM2S263D) mutants, and combining Phos-tag gel electrophoresis, in vitro kinase assays, and mass spectrometry validation, they confirmed Ser263 as a direct phosphorylation site of RPS6KA3. In cellular models using HEK293T, HeLa, and A375SM melanoma cell lines, the subcellular localization and functional differences between wild-type and mutant DRAM2 were systematically analyzed using immunofluorescence colocalization, immunoelectron microscopy, and NTA. At the animal level, a nude mouse xenograft model was used to evaluate the impact of DRAM2 phosphorylation status on tumor growth. Additionally, rps6ka3^+/+ and rps6ka3^-/- MEFs were employed to verify the evolutionary conservation of this pathway in regulating autophagic flux.

Key Conclusions and Perspectives

• RPS6KA3 directly phosphorylates DRAM2 at Ser263, a modification dependent on MAPK pathway activity and enhanced under UVB stimulation — suggesting that extracellular stress signals can regulate autophagy through this axis; it is recommended to assess RPS6KA3 activity and DRAM2 phosphorylation levels in experiments simulating tumor microenvironmental stress.
• Phosphorylated DRAM2 binds to the AP3D1/AP-3 complex via a dileucine motif ([E/D]XXXLØ) adjacent to Ser263 within its C-terminal ERTRLLS sequence, promoting its entry into the late endosome–lysosome pathway — revealing a novel 'phosphorylation–adaptor recognition' regulatory mode; it is recommended to systematically evaluate the functional impact of nearby phosphorylation sites when studying other membrane proteins containing similar sorting motifs.
• The non-phosphorylatable mutant DRAM2S263A fails to bind AP-3, leading to defective lysosomal targeting and instead accumulates near the plasma membrane, enhancing exosome release — indicating that DRAM2's functional fate is determined by its phosphorylation status; it is recommended to examine diversion mechanisms in membrane protein trafficking pathways when studying exosome secretion regulation.
• Loss of RPS6KA3 or DRAM2 significantly inhibits autophagic flux, manifested by impaired LC3B-II accumulation and defective SQSTM1 degradation — supporting this axis as a key positive regulator of late-stage autophagy; it is recommended to include assessment of DRAM2 localization and phosphorylation status alongside classical markers when evaluating autophagy function.
• Clinical data show that RPS6KA3 and DRAM2 are co-overexpressed in melanoma tissues, and high DRAM2 levels correlate with poor patient prognosis — suggesting clinical translational potential for this signaling axis; it is recommended to validate this as a potential biomarker in larger cohorts.

Research Significance and Prospects

This study provides new targets for drug development: targeting the RPS6KA3–DRAM2 interaction may selectively inhibit tumor cell autophagy without completely blocking basal autophagy, thereby reducing toxicity. Moreover, DRAM2 phosphorylation status could serve as a molecular indicator for clinically monitoring autophagy activity, particularly in melanoma patients undergoing targeted or immunotherapy. In terms of disease modeling, generating conditional knock-in DRAM2S263A mutant mice would help recapitulate autophagy-deficient phenotypes in vivo, facilitating the study of tumor microenvironment adaptation mechanisms.

 

 

Conclusion

This study systematically elucidates how RPS6KA3-mediated phosphorylation of DRAM2 acts as a molecular switch between autophagy and exosome secretion, promoting tumor progression in melanoma by ensuring completion of autophagic flux. This discovery not only expands our understanding of the regulatory mechanisms of lysosomal membrane protein trafficking but also highlights the precise control exerted by kinase signaling pathways over organelle dynamics. From bench to bedside, this axis provides a theoretical foundation for developing novel anti-cancer strategies—for example, designing small-molecule inhibitors to block RPS6KA3-mediated phosphorylation of DRAM2, or using DRAM2 phosphopeptides as competitive disruptors. Meanwhile, the subcellular localization of DRAM2 or its enrichment in exosomes may serve as liquid biopsy markers for assessing tumor autophagy status. Future studies could further explore the universality of this pathway in other autophagy-dependent cancers (e.g., pancreatic, lung cancer) and its potential impact on the immune microenvironment. Overall, this study establishes a new cornerstone target for precision therapy in melanoma, advancing a closed-loop research path from mechanistic dissection to translational application.

 

Ga-Eun Lee, Soo-Bin Nam, Eunyoung Moon, Geul Bang, and Cheol-Jung Lee. RPS6KA3/RSK2-mediated phosphorylation of DRAM2 promotes lysosomal targeting and autophagic flux in melanoma. Autophagy.