Drosophila Tpr protein homolog is localized both in the extrachromosomal channel network and to nuclear pore complexes

Here we report structural, molecular, and biochemical characterizations of Bx34, a Drosophila melanogaster nuclear coiled-coil protein which is localized to extrachromosomal and extranucleolar spaces in the nuclear interior and which is homologous to the mammalian nuclear pore complex protein Tpr. In the nuclear interior, Bx34 is excluded from chromosomes and the nucleolus and generally localizes to regions between these structures and the nuclear periphery. This distribution matches the 'extrachromosomal channel network' described previously. In the nuclear periphery, Bx34 localizes on or near nuclear pore complexes. Biochemically, Bx34 isolates exclusively with the nuclear matrix fraction. The Bx34 cDNA sequence predicts a large protein (262 kDa) with two distinct structural domains. The Bx34 N-terminal 70% (180 kDa) is predicted to form an extended region of coiled-coil, while the C-terminal 30% (82 kDa) is predicted to be unstructured and acidic. Bx34 shows moderate sequence identity over its entire length to the mammalian nuclear pore complex protein 'Tpr' (28% amino acid identity and 50% similarity). Furthermore, several of the sequence motifs and biochemical similarities between Bx34 and Tpr are sufficiently striking that it is likely that Bx34 and Tpr are functionally related. The Bx34 gene exists in a single copy in region 48C of chromosome 2R. The localization of coiled-coil Bx34 to both the nuclear interior and nuclear pore complexes and its sequence similarity to a known nuclear pore complex protein leads to speculations about a role for Bx34 in nucleo-cytoplasmic transport which we can test using molecular genetic approaches.