Nucleolar localization and identification of nuclear/nucleolar localization signals of the calmodulin-binding protein nucleomorphin during growth and mitosis in Dictyostelium.

The calmodulin-binding protein nucleomorphin isoform NumA1 is a nuclear number regulator in Dictyostelium that localizes to intra-nuclear patches adjacent to the nuclear envelope and to a lesser extent the nucleoplasm. Earlier studies have shown similar patches to be nucleoli but only three nucleolar proteins have been identified in Dictyostelium. Here, actinomycin-D treatment caused the loss of NumA1 localization, while calcium and calmodulin antagonists had no effect. In keeping with a nucleolar function, NumA1 moved out of the presumptive nucleoli during mitosis redistributing to areas within the nucleus, the spindle fibers, and centrosomal region before re-accumulating in the presumptive nucleoli at telophase. Together, these data verify NumA1 as a true nucleolar protein. Prior to this study, the dynamics of specific nucleolar proteins had not been determined during mitosis in Dictyostelium. FITC-conjugated peptides equivalent to presumptive nuclear localization signals within NumA1 localized to nucleoli indicating that they also act as nucleolar localization signals. To our knowledge, these represent the first precisely defined nucleolar localization signals as well as the first nuclear/nucleolar localization signals identified in Dictyostelium. Together, these results reveal that NumA1 is a true nucleolar protein and the only nucleolar calmodulin-binding protein identified in Dictyostelium. The possible use of nuclear/nucleolar localization signal-mediated drug targeting to nucleoli is discussed.