COILIN CAN FORM A COMPLEX WITH THE U7 SMALL NUCLEAR RIBONUCLEOPROTEIN

Coiled bodies (CBs) in the amphibian oocyte nucleus are spherical stru ctures up to 10 mu m or more in diameter, much larger than their somat ic counterparts, which rarely exceed 1 mu m. Oocyte CBs may have small er granules attached to their surface or embedded within them, which a re identical in structure and composition to the many hundreds of B-sn urposomes found free in the nucleoplasm. The matrix of the CBs contain s the diagnostic protein p80-coilin, which is colocalized with the U7 small nuclear ribonucleoprotein (snRNP), whereas the attached and embe dded B-snurposomes contain splicing snRNPs. A few of the 50-100 CBs in the oocyte nucleus are attached to lampbrush chromosomes at the histo ne gene loci. By coimmunoprecipitation we show that coilin and the U7 snRNP can form a weak but specific complex in the nucleoplasm, which i s dependent on the special U7 Sm-binding site. Under the same conditio ns coilin does not associate with the U1 and U2 snRNPS. Coilin is a nu cleic acid-binding protein, as shown by its interaction with single-st randed DNA and with poly r(U) and poly r(G). We suggest that an import ant function of coilin is to form a transient complex with the U7 snRN P and accompany it to the CBs. In the case of CBs attached to chromoso mes at the histone gene loci, the U7 snRNP is thus brought close to th e actual site of histone pre-mRNA transcription.