Coilin: The first 25 years

Abstract

Initially identified as a marker of coiled bodies (now Cajal bodies or CBs), the protein coilin was discovered a quarter of century ago. Coilin is now known to scaffold the CB, but its structure and function are poorly understood. Nearly devoid of predicted structural motifs, coilin has numerous reported molecular interactions that must underlie its role in the formation and function of CBs. In this review, we summarize what we have learned in the past 25 years about coilin's structure, post-transcriptional modifications, and interactions with RNA and proteins. We show that genes with homology to human coilin are found in primitive metazoans and comment on differences among model organisms. Coilin's function in Cajal body formation and RNP metabolism will be discussed in the light of these developments.

Keywords: Cajal body; coilin; histone; small Cajal body-specific RNA (scaRNA); small nuclear RNA (snRNA); small nuclear ribonucleoprotein particle (snRNP); small nucleolar RNA (snoRNA); spliceosome; splicing; telomere.

Figure 1.

Coilin is highly conserved among metazoans. Predicted coilin amino acid sequences (RefSeq) for the indicated species were aligned with coilin proteins identified by homology search in Cephalochordates (Lancelet), Cnidarians (Hydra) and Placozoa (Trichoplax). Alignments of conserved (A) N-terminal and (B) C-terminal regions are shown, demonstrating the greatest divergence of Drosophila melanogaster coilin from the other species. (C) Cladogram shows numbers of annotated and/or predicted coilin proteins per clade together with hypothetical coilin proteins found by homology search (numbers in brackets).

Figure 2.

Coilin interaction partners and the formation of CBs. (A) Schematic representation of human coilin protein showing key structural features: Tudor domain (purple), RG-box (orange), bipartite nuclear localization signal (blue), cryptic nucleolar localization signal (light blue) and self-interaction domain (green). Black bars signify parts of coilin known to interact with indicated proteins/RNA. (B) The current hypothesis of nuclear body assembly suggests that protein and RNA components (colored shapes) tend to have more interactions (double arrows) among themselves as compared to other molecules (blue circles) in the environment. When the components reach a critical concentration locally, liquid phase separation occurs and creates the nuclear body.