Cajal bodies and coilin--moving towards function

Abstract

Many nuclear factors are concentrated within nonmembrane-bound subnuclear bodies. The Cajal body is an example of a conserved nuclear compartment that has been linked to molecular disease. Recent studies have shown Cajal bodies to be surprisingly mobile and offer clues about their function in the cell.

Figure 1.

Cajal bodies. (A) HeLa cells expressing an FP–coilin fusion protein (arrows indicate Cajal bodies). (B) Fluorescence micrograph of purified CBs immunolabeled with anticoilin antibodies. (C) Field emission scanning electron micrograph of a purified CB. (D) Transmission electron micrograph of a purified CB. Coilin is immunolabeled with 5 nm gold (arrows) and SMN is immunolabeled with 10 nm gold (arrowheads). Bars: (A and B) 10 μm; (C) 500 nm; (D) 200 nm.

Figure 2.

Model of Cajal body function in snRNP/snoRNP biogenesis. (1) U1, U2, U4, and U5 snRNAs are transcribed by RNA polymerase II and exported to the cytoplasm where the trimethyl cap is formed and assembly with the core Sm proteins occurs in the SMN–gemin complex. (2) Partially assembled snRNPs or snoRNPs transit initially through the Cajal body on their maturation pathway, where further RNA modification (2′-O-methylation and pseudouridylation) occurs. (3) Mature snRNPs or snoRNPs exit the Cajal body and move to their site of action. SnoRNPs move to the nucleolus to participate in rRNA modification and ribosome assembly, while snRNPs accumulate in nuclear speckles, and at sites of gene transcription where they function in splicing of premRNA.