COIL

p80-coilin (COIL) is the defining marker protein of the Cajal body (coiled body), a nuclear organelle, and is an integral structural component required for the organization of this compartment and its recruitment of splicing snRNPs (PMID:2033369, PMID:7490287). Coilin localization is governed by separable domains: its amino-terminal region (first ~102 amino acids) is necessary and sufficient for targeting to the coiled-body organelle, acting independently of the nuclear localization signal, while the carboxy-terminal domain is required to recruit splicing snRNPs—deletion of the C-terminus produces pseudo-coiled bodies and eliminates endogenous coiled bodies (PMID:7532471, PMID:7490287). Internal domains of coilin functionally couple it to nucleolar architecture, since their mutation generates perinucleolar fibrillar structures, segregates nucleolar antigens, and abolishes RNA polymerase I activity (PMID:7490287). Beyond coiled bodies, coilin is a nuclear-matrix-associated constituent of interchromatin granule-associated zones (PMID:7622600), and it physically interacts with ataxin-1 (the SCA1 gene product) through the C-terminal regions of both proteins, co-localizing with ataxin-1 aggregates in the nucleoplasm (PMID:12757932). The ubiquitously expressed protein is encoded by a 7-exon gene on chromosome 17q22-23 (PMID:7971277, PMID:7490287).

1. 1991 High

   Established the existence of a dedicated coiled-body marker protein, defining the molecular identity of this previously ultrastructurally-described nuclear organelle.

   Evidence Autoantibody-based immunoblot, immunofluorescence, immunoelectron microscopy, and cDNA expression cloning identifying an 80-kDa coiled-body protein

   PMID:2033369

   Open questions at the time

   • Function within the coiled body not yet defined
   • Full-length sequence and domain organization not yet resolved
2. 1994 High

   Mapped a discrete N-terminal targeting determinant, showing coiled-body localization is encoded separately from nuclear import.

   Evidence Microinjection of myc-tagged coilin mRNA and deletion constructs into Xenopus oocytes with germinal-vesicle immunofluorescence

   PMID:7532471

   Open questions at the time

   • Molecular partner that reads the N-terminal targeting signal unknown
   • Mechanism of organelle assembly versus targeting not distinguished
3. 1994 Medium

   Defined the gene locus and full-length protein architecture, providing the structural framework of two conserved N- and C-terminal domains.

   Evidence 5'-RACE, genomic library screening, Northern blot, and FISH mapping to chromosome 17q22-23

   PMID:7971277

   Open questions at the time

   • Functional roles of the conserved domains not assigned here
   • Discrepancy between predicted mass and apparent 80-kDa size unexplained
4. 1995 High

   Assigned distinct functions to coilin domains, separating snRNP recruitment from nucleolar/Pol I coupling and revealing coilin as an organizer of multiple nuclear compartments.

   Evidence Systematic deletion/truncation mutagenesis in HeLa cells with snRNP co-localization, nucleolar antigen imaging, and RNA polymerase I activity assays

   PMID:7490287

   Open questions at the time

   • Direct biochemical mechanism linking coilin to Pol I activity not established
   • Identity of snRNP-recruiting binding partners not defined
5. 1995 Medium

   Extended coilin localization to interchromatin granule-associated zones as a nuclear-matrix-associated component, broadening its structural role beyond coiled bodies.

   Evidence Immunoelectron microscopy after low-salt and high-salt/DNase nuclear matrix extraction with snRNA in situ hybridization

   PMID:7622600

   Open questions at the time

   • Functional consequence of IGAZ localization unknown
   • Single-lab observation by IEM only
6. 2003 Medium

   Identified a direct protein partner, ataxin-1, linking coilin to a neurodegenerative disease protein and to nucleoplasmic aggregate biology.

   Evidence Yeast two-hybrid, reciprocal co-immunoprecipitation, deletion mapping, and co-localization in HeLa cells and SCA1 transgenic mouse Purkinje cells

   PMID:12757932

   Open questions at the time

   • Functional consequence of coilin-ataxin-1 binding for either coiled bodies or SCA1 pathology not established
   • Single-lab interaction
7. 2006 Low

   Used coilin as a coiled-body marker to place CGI-55 in this compartment, indicating the coiled body recruits additional nuclear-body-associated proteins.

   Evidence Yeast two-hybrid screen and GFP/RFP-fusion co-localization imaging in HeLa cells

   PMID:16679534

   Open questions at the time

   • Co-localization only; no direct binding assay between CGI-55 and coilin
   • Coilin studied as a marker, not as the functional subject

• The biochemical mechanism by which coilin assembles and maintains Cajal bodies, recruits snRNPs, and couples to RNA polymerase I activity remains undefined.
• No structural model of the coilin-driven assembly
• Direct molecular partners for snRNP recruitment and Pol I coupling unidentified
• Physiological role of ataxin-1 interaction unresolved