A DROSOPHILA TPR PROTEIN HOMOLOG IS LOCALIZED BOTH IN THE EXTRACHROMOSOMAL CHANNEL NETWORK AND TO NUCLEAR-PORE COMPLEXES

Here we report structural, molecular, and biochemical characterization s of Bx34, a Drosophila melanogaster nuclear coiled-coil protein which is localized to extrachromosomal and extranucleolar spaces in the nuc lear interior and which is homologous to the mammalian nuclear pore co mplex protein Tpr. In the nuclear interior, Bx34 is excluded from chro mosomes and the nucleolus and generally localizes to regions between t hese structures and the nuclear periphery, This distribution matches t he 'extrachromosomal channel network' described previously, In the nuc lear periphery, Bx34 localizes on or near nuclear pore complexes, Bioc hemically, Bx34 isolates exclusively with the nuclear matrix fraction, The Bx34 cDNA sequence predicts a large protein (262 kDa) with two di stinct structural domains. The Bx34 N-terminal 70% (180 kDa) is predic ted to form an extended region of coiled-coil, while the C-terminal 30 % (82 kDa) is predicted to be unstructured and acidic. Bx34 shows mode rate sequence identity over its entire length to the mammalian nuclear pore complex protein 'Tpr' (28% amino acid identity and 50% similarit y), Furthermore, several of the sequence motifs and biochemical simila rities between Bx34 and Tpr are sufficiently striking that it is likel y that Bx34 and Tpr are functionally related, The Bx34 gene exists in a single copy in region 48C of chromosome 2R, The localization of coil ed-coil Bx34 to both the nuclear interior and nuclear pore complexes a nd its sequence similarity to a known nuclear pore complex protein lea ds to speculations about a role for Bx34 in nucleo-cytoplasmic transpo rt which we can test using molecular genetic approaches.