Figure 1.
U3 snoRNP proteins shuttle between nuclei and the cytoplasm at different rates. (A) Untransfected HeLa were fused in heterokaryon assays with HeLa transfected with constructs encoding either negative (GFP-UBF) or positive (GFP-PTB) controls, U3 snoRNP core proteins GFP-fibrillarin or -U3-55K (left) or the preribosome-associated proteins GFP-Imp3, -Imp4, -Mpp10, -Rcl1, or -Sof1 (right). Two cell heterokaryons were analyzed 2 h after fusion. DNA was labeled with DAPI to delineate nuclei and nucleoli. Thick arrows indicate nucleoli of transfected cells, thin arrows the nucleoli of untransfected cells. Bar, 10 μm. (B) Quantitative analyses of the heterokaryon assays represented in A were done to compare the relative shuttling rates of the proteins analyzed in A. For all constructs besides GFP-PTB, the FR (y-axis) was measured as fluorescence intensity in the untransfected cell nucleolus (FU)/fluorescence intensity in the transfected cell nucleolus (FT) within a same-sized area. For GFP-PTB, nucleoplasmic rather than nucleolar fluorescence intensity was used.