RTL5 (retrotransposon Gag-like 5; also SIRH8, RGAG4, Mart5) is a eutherian-specific, X-linked gene domesticated from the gag (capsid) region of a Ty3/gypsy (sushi-ichi-related) LTR retrotransposon. It encodes a large, strongly acidic protein with a single central retroviral capsid-like domain flanked by long disordered acidic segments, and it has lost the retroelement nucleocapsid zinc knuckle, protease and retrotransposition machinery. In the mouse brain RTL5 is produced by microglia and appears as extracellular dots and granules; it rapidly accumulates on injected double-stranded RNA analog (and on unmethylated CpG DNA) and Rtl5 knockout mice show delayed removal of dsRNA, whereas LPS clearance, which depends on its basic paralog RTL6, is unaffected. RTL5 therefore acts as an extracellular, microglia-derived innate immune factor that helps clear virus-like nucleic acid PAMPs from the brain. It is also expressed in anterior pituitary corticotrophs, where it is a transcriptional target of Ikaros and its knockdown in a corticotroph cell model reduces hormone gene expression and ACTH secretion. The biochemical activity underlying these roles (direct nucleic acid binding, self-assembly) has not been defined.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003674 molecular_function | ND GO_REF:0000015 | ACCEPT | Summary: ND root molecular function annotation. No biochemical activity of RTL5 has been defined. Reason: Literature search confirms that the molecular activity of RTL5 is still unknown: the knockout and knock-in work defines a role in dsRNA removal in the brain but explicitly does not resolve whether RTL5 binds dsRNA directly or how removal is achieved. The ND root term remains the correct statement for MF. Supporting Evidence: PMID:36162816 However, it should be noted that the confocal fluorescence microscopy data in this study are qualitative, and further study using quantitative methods will be required to examine the precise pathogen removal activities as well as the interaction between RTL5 and RTL6. file:mouse/Rtl5/Rtl5-uniprot.txt They do not correspond to functional retrotransposons, but rather to neofunctionalized retrotransposons genes. file:mouse/Rtl5/Rtl5-deep-research-openscientist.md retaining the Gag capsid fold but having lost both the ancestral aspartic-protease active site and the Gag nucleocapsid CCHC zinc-knuckle |
| GO:0005575 cellular_component | ND GO_REF:0000015 | MODIFY | Summary: ND root cellular component annotation, now superseded by in vivo localization data. Reason: Knock-in RTL5-mCherry mice show the protein in microglia and as extracellular dots and granules in the CNS; the RTL9 paper from the same group describes RTL5 in microglial secretory granules that are secreted into the extracellular space. Extracellular region is the supported location where RTL5 acts. Proposed replacements: extracellular region Supporting Evidence: PMID:36162816 Venus and mCherry knock-in mice exhibited expression of RTL5-mCherry and RTL6-Venus fusion proteins in microglia and appeared as extracellular dots and granules in the central nervous system PMID:37834332 The RTL5 and RTL6 proteins in the secretory granules of microglia are secreted into the extracellular space and there await their targets, such as dsRNA and LPS |
| GO:0008150 biological_process | ND GO_REF:0000015 | MODIFY | Summary: ND root biological process annotation, now superseded by knockout data. Reason: Rtl5 knockout mice show significantly delayed removal of an injected dsRNA analog from the brain, while LPS removal is unaffected, and RTL5 accumulates on the injected dsRNA as granules. RTL5 is a pre-made, secreted extracellular factor that captures the PAMP (dsRNA, a viral nucleic-acid mimic) and is required for its clearance -- an effector of an antiviral defense response rather than a cell changing state in response to the stimulus, so defense response to virus fits better than a cellular-response term. The pituitary ACTH phenotype is kept out of GO pending more than knockdown data. Proposed replacements: defense response to virus Supporting Evidence: PMID:36162816 Experiments using Rtl6 or Rtl5 knockout mice provided additional evidence that RTL6 and RTL5 act as factors against LPS and dsRNA, respectively, in the brain PMID:36162816 indicating that, without RTL5, dsRNA removal was significantly delayed in the brain PMID:36162816 In the case of LPS injection in the Rtl5 KO mice, the LPS removal activity was essentially unaffected. PMID:42511665 These findings suggest that SIRH3/RTL6 and SIRH8/RTL5 contribute to the elimination of bacterial- and viral-derived PAMPs from the brain |
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Download this section (compressed HTML)Q: Does RTL5 bind dsRNA directly (and with what affinity and length/sequence preference), or does it accumulate on dsRNA only as part of larger RTL5/RTL6 assemblies?
Q: How is RTL5, which lacks a signal peptide, released from microglia into the extracellular space?
Q: Is the pituitary ACTH phenotype of Rgag4/Rtl5 knockdown reproduced in Rtl5 knockout mice, and is it related to the microglial function?
Q: Does RTL5 retain any capsid-like self-assembly, given its single ordered Gag capsid-like domain flanked by long acidic disordered regions?
Experiment: Recombinant RTL5 (full length and capsid-like domain) binding assays with dsRNA, poly(I:C), ssRNA and CpG DNA by EMSA or fluorescence anisotropy, plus negative-stain EM for higher-order assemblies.
Experiment: Quantitative clearance kinetics of labeled poly(I:C) and live RNA-virus challenge in Rtl5 KO versus wild-type brain, with readouts of interferon response and viral load.
Experiment: Pituitary hormone (ACTH, POMC) measurements and stress-axis testing in Rtl5 knockout mice.
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