Introduction: Histone-lysine N-methyltransferase 2D (KMT2D) is an H3K4 methyltransferase and a potential tumor suppressor with a crucial role in regulating gene expression. Its dysregulation has been implicated in developmental disorders and several types of cancers. Despite this, the molecular mechanisms that govern its activity remain largely elusive. Among these, post-translational modifications, especially phosphorylation, serve as an essential regulator, fine-tuning KMT2D stability, localization and functional interactions for maintaining cellular homeostasis. With over 173 phosphorylation sites reported, KMT2D is significantly regulated by kinases and exploring its phospho-regulatory network based on targeted in vitro approaches is challenging. Methods: We systematically curated and integrated the global phosphoproteomic datasets, along with their corresponding experimental conditions, to comprehensively identify the phosphorylation events reported for KMT2D. The site exhibiting the highest frequency of detection across these datasets is considered the predominant phosphorylation site. To investigate its functional significance, we analyzed the proteins and their phosphorylation sites that are differentially co-regulated with the predominant site, as well as its associated upstream kinases and interacting proteins. Results: Among the 173 reported phosphorylation sites representing KMT2D, Serine 2274 (S2274) emerged as the predominant site being detected in over 42% of diverse mass spectrometry-based phosphoproteomics datasets. This site lies within one of KMT2D’s unique “ LSPPP ” motifs, suggesting a potential regulatory role. Detailed investigation on the differentially co-regulated protein phosphosites revealed the phosphorylation of KMT2D at S2274 is consistently and positively co-regulated with MAPK1/ERK2 activation, as well as with the proteins involved in the MAPK cascade, epigenetic regulation and cell differentiation. Notably, ERK2 was predicted as an upstream kinase targeting S2274, suggesting that KMT2D S2274 functions as a potential downstream effector of MEK-ERK signaling pathway, potentially linking to epigenetic regulation and cell differentiation. Further, our results highlighted a potential mechanistic link between disrupted phosphorylation at S2274 and the pathogenesis of Kabuki syndrome. Discussion: This study delineates the phosphoregulatory network of KMT2D, positioning it as a dynamic epigenetic effector modulated by MEK-ERK signaling, with broader implications for cancer and developmental disorders.

Protein Kinase N1 (PKN1) is a PKC-related serine/threonine kinase of the AGC group within the eukaryotic protein kinase superfamily (ePK) that orchestrates oncogenic, metabolic, and cytoskeletal signaling. Despite these critical roles, the phosphorylation-dependent regulatory network of PKN1 remains largely undefined. We performed a large-scale phosphoproteomic data integration of publicly available human datasets (892 profiling datasets and 191 differential datasets) to identify recurrent PKN1 phosphorylation sites. This analysis identified two predominant PKN1 phosphosites, S562 and S916, that were frequently observed and differentially regulated across studies. The S916 maps to a turn motif (TM) in the AGC group of kinases, which is evolutionarily conserved among PKN paralogs, while S562 is non-conserved and appears to be PKN1-specific. Co-regulation and enrichment analyses suggest that S916 is associated with insulin/AMPK signaling and metabolic pathways, whereas S562 co-occurs with phosphosites involved in cell division, cytoskeletal regulation, and microtubule cytoskeleton organization. Integrating predicted and experimentally validated kinases, substrates, and interactors, we reconstructed a phospho-regulatory network that positions PKN1 at the crossroads of cytoskeleton organization and metabolic signaling. To assess the disease relevance of these phosphorylation events, we integrated transcriptomic and phosphoproteomic data from the hepatocellular carcinoma database (HCCDB). PKN1 was markedly up-regulated in HCC, and its phosphorylation at S916 was positively co-regulated with multiple oncogenic and proliferation-associated protein phosphosites. These results predict S562 and S916 as potential sites for targeted biochemical validation and functional experiments. The identification of S562 and S916 as key regulatory sites provides new mechanistic insight into PKN1 activation and highlights potential avenues for therapeutic targeting.

Unc-51-like autophagy-activating kinase 1 (ULK1), a serine/threonine kinase, is the central regulator of autophagy initiation and plays a crucial role in cellular homeostasis, metabolic adaptation, and stress responses. We performed a large-scale phosphoproteomic data analysis of ULK1 by systematically integrating and analyzing 891 profiling and 245 differential human phosphoproteomic datasets. S450, S623, and S638 emerged as the predominant phosphosites in ULK1, with the highest frequency of detection across multiple cellular phosphoproteomics datasets. Conservation analysis showed that these phosphosites are conserved across mammalian species and human ULK1 and ULK2 (Unc-51-like autophagy-activating kinase 2). The co-regulatory networks associated with these phosphosites, including interacting proteins, upstream kinases, and downstream substrates, were systematically analyzed. The phosphosite co-regulated ULK1 interactome revealed a network of associations linking ubiquitin-dependent cargo recognition to autophagosome biogenesis, trafficking, and maturation, indicating that ULK1 phosphoregulation may extend beyond its established role in autophagy initiation and be associated with multiple stages of selective autophagy. Notably, ULK1 phosphosites were linked to key metabolic enzymes in glycogen, sphingolipid, phospholipid, and glycolytic pathways, as well as classical lipolysis. Protein-normalized analysis across 12 cancer cohorts showed significant upregulation of the predominant ULK1 phosphosites, most frequently at S450 and S623. Comparative analysis with an independent ULK1/2 knockout phosphoproteomic dataset further supported the relevance of these identified co-regulated protein phosphosites. Collectively, this study provides a phosphosite-resolved view of ULK1 regulation and identifies candidate phosphosites and associated proteins for future functional and mechanistic studies.