ZCCHC7

ZCCHC7 is the human orthologue of yeast Air1 and functions as the Air component of a human TRAMP-like complex that couples RNA polyadenylation to nuclear RNA quality control (PMID:21878619). It localizes to the nucleolus and interacts with the two mammalian Trf4 orthologues, the non-canonical poly(A) polymerases PAPD5 and PAPD7 (PMID:21878619). This interaction is mediated by a conserved IWRXY motif located in the linker between zinc knuckles 4 and 5, a structural element shown in the yeast Air1/Air2 orthologues to be critical for Trf4 binding and TRAMP complex integrity and conserved in human ZCCHC7 (PMID:21878619). Beyond its identification as the Air subunit of a human TRAMP complex and the mapping of the IWRXY interaction motif by orthology, no further mechanistic detail — including direct mutagenesis of the human protein or functional reconstitution — has been characterized in the available corpus.

1. 2011 Medium

   Whether humans possess an Air-type adaptor to organize a TRAMP-like RNA surveillance complex was unresolved; identifying ZCCHC7 as nucleolar and as a PAPD5/PAPD7 partner established it as the human Air component coupling polyadenylation to nuclear RNA quality control.

   Evidence Immunofluorescence localization and co-immunoprecipitation with PAPD5/PAPD7 in human cells, with sequence analysis identifying a conserved IWRXY motif

   PMID:21878619

   Open questions at the time

   • No mutagenesis of human ZCCHC7 to confirm the IWRXY motif mediates PAPD5/PAPD7 binding directly
   • No functional reconstitution of a human TRAMP complex or demonstration of RNA quality control activity
   • RNA substrates targeted by the human complex not identified
2. 2011 Medium

   How the Air adaptor docks onto the poly(A) polymerase was unknown; genetic and biochemical dissection in yeast showed zinc knuckles 4-5 and a conserved IWRXY linker motif mediate the Trf4 interaction and TRAMP integrity, and the conservation of this motif in ZCCHC7 implicates the same element in its PAPD5/PAPD7 binding.

   Evidence Random and targeted zinc-knuckle mutagenesis, functional complementation, CUT stabilization, and TRAMP integrity assays in yeast Air1/Air2; sequence conservation in human ZCCHC7

   PMID:21878619

   Open questions at the time

   • Human ZCCHC7 IWRXY requirement inferred from yeast and sequence conservation, not directly tested
   • Structural basis of the ZCCHC7-PAPD5/PAPD7 interface not determined
   • Contribution of individual zinc knuckles to human complex assembly unmapped

• The RNA substrate spectrum, catalytic contribution within the human complex, and the link to nuclear exosome-mediated degradation remain to be established experimentally for human ZCCHC7.
• No identified RNA substrates of the human TRAMP-like complex
• No direct demonstration of coupling to the nuclear exosome in human cells
• No structural or reconstitution data for the human complex