Squid hnRNP protein promotes apical cytoplasmic transport and localizationof Drosophila pair-rule transcripts

Drosophila melanogaster pair-rule segmentation gene transcripts localize ap ically of nuclei in blastoderm embryos. This might occur by asymmetric (vec torial) export from one side of the nucleus or by transport within the cyto plasm. We have followed fluorescently labeled pair-rule transcripts postinj ection into Drosophila embryos. Naked, microinjected fushi tarazu (ftz) tra nscripts do not localize in blastoderm embryos, indicating that cytoplasmic mechanisms alone are insufficient for epical targeting. However, prior exp osure of ftz to Drosophila or human embryonic nuclear extract leads to rapi d, specific, microtubule-dependent transport, arguing against vectorial exp ort. We present evidence that ftz transcript localization involves the Squi d (Hrp40) hnRNP protein and that the activity of hnRNP proteins in promotin g transcript localization is evolutionarily conserved. We propose that cyto plasmic localization machineries recognize transcripts in the context of nu clear partner proteins.