In the larval salivary glands of C. tentans, it is possible to visualize by electron microscopy how Balbiani ring (BR) pre-mRNA associates with proteins to form pre-mRNP particles, how these particles move to and through the nuclear pore, and how the BR RNA is engaged in the formation of giant polysomes in the cytoplasm. Here, we study C. tentans hrp36, an abundant protein in the BR particles, and establish that it is similar to the mammalian hnRNP A1. By immuno-electron microscopy it is demonstrated that hrp36 is added to BR RNA concomitant with transcription, remains in nucleoplasmic BR particles, and is translocated through the nuclear pore still associated with BR RNA. It appears in the giant BR RNA-containing polysomes, where it remains as an abundant protein in spite of ongoing translation.

Abstract. In vertebrates, a nuclear cap-binding complex (CBC) formed by two cap-binding proteins, CBP20 and CBP80, is involved in several steps of R N A metabolism, including pre-mRNA splicing and nuclear export of some R N A polymerase II-transcribed U snRNAs. The CBC is highly conserved, and antibodies against human CBP20 cross-react with the CBP20 counterpart in the dipteran Chironomus tentans. Using immunoelectron microscopy, the in situ association of CBP20 with a specific pre-mRNP particle, the Balbiani ring particle, has been analyzed at different stages of pre-mRNA synthesis, maturation, and nucleo-cytoplasmic transport. We demonstrate that CBP20 binds to the nascent pre-mRNA shortly after transcription initiation, stays in the RNP particles after splicing has been completed, and remains attached to the 5' domain during translocation of the RNP through the nuclear pore complex (NPC). The rapid association of CBP20 with nascent RNA transcripts in situ is consistent with the role of CBC in splicing, and the retention of CBC on the RNP during translocation through the NPC supports its proposed involvement in RNA export. cursors (pre-mRNAs) undergo a series of maturation -~ reactions before they are exported to the cytoplasm where the mature messenger RNAs (mRNAs) can direct protein synthesis. One of these maturation events is the addition of a monomethylated guanosine residue to the first encoded nucleotide of the RNA via a 5'-5' triphosphate linkage (Shatkin, 1976). This reaction occurs on all transcripts synthesized by RNA polymerase II shortly after the start of transcription (Salditt-Georgieff et al., 1980;Rasmussen and Lis, 1993). The resulting cap structure is implicated in several aspects of pre-mRNA and mRNA metabolism. The cap structure increases RNA stability, and is required for efficient translation initiation, premRNA splicing, and nuclear export of capped RNAs (Furuichi et al

Correspondence: Neus.Visa@molbio.su.se and Piergiorgio.Percipalle@ki.se Actin participates in several essential processes in the cell nucleus. Even though the presence of actin in the nucleus was proposed more than 30 years ago, nuclear processes that require actin have been only recently identified. Actin is part of chromatin remodeling complexes; it is associated with the transcription machineries; it becomes incorporated into newly synthesized ribonucleoproteins; and it influences long-range chromatin organization. As in the cytoplasm, nuclear actin works in conjunction with different types of actin-binding proteins that regulate actin function and bridge interactions between actin and other nuclear components.

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