Identification of La ribonucleoproteins as a component of interchromatin granules.

Monoclonal antibodies raised against the La antigen were used to localize by preembedding immunoelectron microscopy, snRNPs containing this protein. The results demonstrate that La RNPs are localized in clusters of interchromatin granules, both in Triton X-100-extracted and DNase-digested nuclei. DNase-digested salt-extracted nuclei contained, in addition, labeled structures identified as perichromatin granules and fibers. A close association of labeled granules with the nucleoli was also observed. Digestion of nuclei with DNase yielded residual scaffolds of intermediate filaments and nuclear lamina devoid of interchromatin granules and La immunostaining. Release of the La antigen was tested in the presence of ATP and cytochalasin B. Only cytochalasin was effective suggesting a role for nuclear actin in anchorage of snRNPs.