Table I

Characteristics of the GFP–PTB Deletion Mutant Fusion Proteins

PTBsGFP–PTBs in the PNCEndogenous PTB in the PNCFibrillarinSC35NucleusCytoplasm
GFP–PTB+++++++++++−−−−
GFP–PTB 1++++++++++−−−−
GFP–PTB 2−−−−+++++++    −−
GFP–PTB 3−−−−+++++++   +
GFP–PTB 4+++++++++++   ++
GFP–PTB 5−−−−++++   +++
GFP–PTB 6−−−−+++++++   ++
GFP–PTB 7−−−−++++++   ++
GFP–PTB 8−−−−+++++++   +++
GFP–PTB 9−−−−+++++++   +++
  • The number of pluses and minuses indicates a rough quantitation of each activity by fluorescence intensity. (The single pluses in columns 2–4 indicate that significant changes were observed.) It is clear that at least three RRMs are required for a proper targeting of the fusion proteins to the PNC (column 1). The expression of either the wild-type or the mutant fusion proteins does not affect the pre-existing PNCs (column 2, which shows the localization of the PNC by SH54), the localization of nucleolar protein fibrillarin (column 3), or the splicing factor, SC35 (column 4). In addition, the deletion mutants that contain the NH2 terminus of the PTB are localized predominantly in the nucleus (column 4), whereas the mutants that do not contain the NH2 terminus of the protein show a cytoplasmic localization tendency (column 5).