Appearance and origin of snRNP antigens in chick erythrocyte nuclei reactivated in heterokaryons

Fusion of terminally differentiated chick erythrocytes (CE) with transcriptionally active rat myoblasts results in heterokaryons in which the CE nuclei undergo reactivation of RNA synthesis and splicing. In order to analyze the transport and assembly of small nuclear ribonucleoprotein (snRNP) particles and larger molecular complexes engaged in RNA processing, we have examined CE nuclei in heterokaryons for the presence of four U snRNP-related nuclear antigens (Sm, 70,000 Mr, F78 and M3G-cap) and for one antigen (La), associated with RNA polymerase III transcripts. Inactive erythrocyte nuclei showed low levels of Sm and F78 antigens, but the other antigens were undetectable. Immediately after fusion, the fluorescence of the pre-existing chicken Sm antigen was detected in the CEn, and then the intensity of the signal increased rapidly during reactivation. The other antigens appeared more slowly, reaching full intensity at different time points after fusion. Blocking of chick transcription did not block the appearance of Sm, 70,000 Mr, cap and La antigens but did effectively inhibit the appearance of the F78 antigen. It has previously been demonstrated that the structure recognized by this monoclonal antibody is physically associated with functional splicing complexes. Blocking of translation in heterokaryons abolished uptake of snRNPs into the chicken nuclei. Taken together, the results indicate that rat snRNP complexes were imported into the chick nuclei after assembly in the cytoplasm. For all the studied antigens, except F78, this translocation was independent of chick RNA synthesis. The appearance of the F78 antigen was totally dependent on expression of chicken genes.