{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\nPCIF1 was first characterized as a WW domain‐containing protein that directly binds the serine‐5 phosphorylated C-terminal domain of RNA polymerase II, thereby localizing to sites of active transcription and contributing to the regulation of pre-mRNA processing. Its association with hyperphosphorylated polymerase and modulation of CTD phosphatase activity suggest that PCIF1 helps coordinate transcriptional elongation and RNA processing events."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "1", "end_ref": "3"}]}, {"type": "t", "text": "\n"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nSubsequent studies established that PCIF1 is the sole cap-specific adenosine methyltransferase responsible for catalyzing N6-methylation of 2′-O-methyladenosine (m6Am) at the very 5′ end of capped mRNAs. Structural, biochemical, and transcriptome-wide mapping analyses have defined its unique substrate specificity and kinetic properties, demonstrating that this modification can influence mRNA translation and stability while being functionally distinct from internal m6A methylation."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "4", "end_ref": "8"}]}, {"type": "t", "text": "\n"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nIn addition to its role in modifying cellular mRNA, PCIF1 also methylates the cap structures of viral mRNAs, thereby generating m6Am on these transcripts. This modification has been shown to attenuate the antiviral effects of interferon signaling, suggesting that by modifying viral RNAs, PCIF1 can modulate host-pathogen interactions and influence the outcome of viral infections."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "9"}]}, {"type": "t", "text": "\n"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nFurther biochemical investigations have highlighted that PCIF1 also exhibits ancillary methylation activities on internal adenosines, albeit at lower catalytic efficiencies compared to its robust activity on cap-adjacent Am. These findings, together with detailed enzymological studies, underscore the enzyme’s broader substrate recognition and binding properties, expanding our understanding of its role in RNA modification."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "11"}]}, {"type": "t", "text": "\n"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nMoreover, emerging evidence implicates PCIF1 in diverse pathophysiological settings. While its precise impact on glioma cell proliferation remains under investigation, increased PCIF1 expression in colorectal cancer correlates with enhanced tumor growth and resistance to immunotherapy, and its cooperation with cofactors has been linked to oncogenic signaling in head and neck squamous cell carcinoma. In parallel, PCIF1-mediated m6Am modification plays a role in the regulation of ciliogenesis and may contribute to chemoresistance in non-small cell lung cancer, emphasizing the broad biological and clinical significance of this enzyme."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "13", "end_ref": "17"}]}, {"type": "t", "text": "\n"}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "Hong Fan, Kazumi Sakuraba, Akihiko Komuro, et al. 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