Zfrp8 forms a complex with fragile-X mental retardation protein and regulates its localization and function

Abstract

Fragile-X syndrome is the most commonly inherited cause of autism and mental disabilities. The Fmr1 (Fragile-X Mental Retardation 1) gene is essential in humans and Drosophila for the maintenance of neural stem cells, and Fmr1 loss results in neurological and reproductive developmental defects in humans and flies. FMRP (Fragile-X Mental Retardation Protein) is a nucleo-cytoplasmic shuttling protein, involved in mRNA silencing and translational repression. Both Zfrp8 and Fmr1 have essential functions in the Drosophila ovary. In this study, we identified FMRP, Nufip (Nuclear Fragile-X Mental Retardation Protein-interacting Protein) and Tral (Trailer Hitch) as components of a Zfrp8 protein complex. We show that Zfrp8 is required in the nucleus, and controls localization of FMRP in the cytoplasm. In addition, we demonstrate that Zfrp8 genetically interacts with Fmr1 and tral in an antagonistic manner. Zfrp8 and FMRP both control heterochromatin packaging, also in opposite ways. We propose that Zfrp8 functions as a chaperone, controlling protein complexes involved in RNA processing in the nucleus.

Tsr4 Is a Cytoplasmic Chaperone for the Ribosomal Protein Rps2 in Saccharomyces cerevisiae

“…Tsr4 is evolutionarily conserved among eukaryotes (30). PDCD2/Zfrp8 in Drosophila directly interacts with Rps2, is both cytoplasmic and nuclear, and appears to serve a function in 40S production, but the significance of its interaction with Rps2 has not been extensively characterized (29,41,49). Humans have two Tsr4 homologs, PDCD2 and PDCD2L (30), and expression of human PDCD2 can fully complement the loss of Drosophila PDCD2/Zfrp8, indicating functional conservation (49).…”

“…In a more recent study, the Bachand group showed that PRMT3 does not cotranslationally associate with Rps2 (56), but whether or not PDCD2L or PDCD2 cotranslationally associates with Rps2 was not explored. Notably, Drosophila PDCD2/Zfrp8 copurifies with translation machinery, suggesting that cotranslational capture of Rps2 may be a conserved function of Tsr4 orthologs (41). Interestingly, the human zinc finger protein ZNF277 was identified as a cotranslational interactor of Rps2; however, knockdown of ZNF277 does not impair 40S production, suggesting that the ZNF277-Rps2 complex has a role outside ribosome biogenesis (56).…”

“…Dedicated RP chaperones typically remain bound to their client RPs during nuclear import (13,15,17,18) to prevent their interaction with RNA cargo being exported from the nucleus and to facilitate loading of the RP onto preribosomes once inside the nucleus. Moreover, metazoan Tsr4 orthologs appear to shuttle between the nucleus and cytoplasm (28,41). Tsr4 contains a weak putative, canonical bipartite nuclear localization signal (NLS) sequence ( Fig.…”

Zfrp8/PDCD2 Interacts with RpS2 Connecting Ribosome Maturation and Gene-Specific Translation

“…The mislocalization of BicD and Orb as well as the severe abnormalities of the Zfrp8 germ line may result from abnormal localization of their mRNA and defects in spatial translation, suggesting that Zfrp8/PDCD2 may be immediately involved in mRNA transport. Our recent studies show that PDCD2/Zfrp8 interacts with a number of proteins responsible for mRNA localization and translational repression [ 14 ] such as FMRP/Fmr1 (Fragile-X Mental Retardation Protein [ 15 ], and NUFIP1/Nufip (Nuclear FMRP-interacting Protein, [ 14 , 16 ]). FMRP has been shown to associate with NUFIP and with mRNAs in the nucleus and this RNP complex is exported into the cytoplasm, where FMRP regulates mRNA translation and degradation [ 17 – 19 ].…”

“…FMRP has been shown to associate with NUFIP and with mRNAs in the nucleus and this RNP complex is exported into the cytoplasm, where FMRP regulates mRNA translation and degradation [ 17 – 19 ]. Zfrp8 directly binds Nufip, is required for the proper localization of FMRP to cytoplasmic puncta, and likely affects the same or overlapping RNA/mRNA targets as FMRP [ 14 ]. The genetic interaction between Fmr1 and Zfrp8 shows that they have antagonistic functions [ 14 ].…”