The nuclear localization of Drosophila Hsp27 is dependent on a monopartite arginine-rich NLS and is uncoupled from its association to nuclear speckles

Drosophila Hsp27 is a small heat shock protein displaying exclusive nuclear localization both before and after heat shock. However, the mechanism implicated in this nuclear localization as well as the required sequences, are undefined. This study identifies the Hsp27 sequences mediating its nuclear localization. The generation of chimeric fusions between Hsp27 and Hsp23, a small heat shock protein displaying exclusive cytoplasmic localization, delineated a stretch of 15 amino acids containing a nuclear-targeting activity. Site-directed mutagenesis within this region unveiled the implication of three arginine residues (R54-R55-R56), which differentially combine to form a novel kind of nuclear localization signal (NLS). Abrogation of the nuclear localization signal activity indicated that Drosophila Hsp27 could still enter the nucleus to associate with nuclear speckles in a NLS-independent fashion. Mutagenesis of a putative nuclear export signal unveiled two leucine residues (L50 and L52) specifically involved in the association of Hsp27 to nuclear speckles and revealed novel nuclear structures formed by this Hsp27 mutant. The present study identifies two distinct sets of sequences respectively mediating the nuclear import of Hsp27 and its association to nuclear speckles. These two phenomena are uncoupled and can be separately abrogated.