Nuclear trafficking of La protein depends on a newly identified nucleolar localization signal and the ability to bind RNA.

Here we provide evidence for an interaction-dependent subnuclear trafficking of the human La (hLa) protein, known as transient interaction partner of a variety of RNAs. Among these, precursor transcripts of certain RNAs are located in the nucleoplasm or nucleolus. Here we examined which functional domains of hLa are involved in its nuclear trafficking. By using green fluorescent-hLa fusion proteins, we discovered a nucleolar localization signal and demonstrated its functionality in a heterologous context. In addition, we revealed that the RRM2 motif of hLa is essential both for its RNA binding competence in vitro and in vivo and its exit from the nucleolus. Our data imply that hLa traffics between different subnuclear compartments, which depend decisively on a functional nucleolar localization signal as well as on RNA binding. Directed trafficking of hLa is fully consistent with its function in the maturation of precursor RNAs located in different subnuclear compartments.