The graded distribution of the dorsal morphogen is initiated by selective nuclear transport in Drosophila

Abstract

The maternal morphogen dorsal (dl) plays a key role in the establishment of dorsal-ventral polarity in Drosophila. We present evidence that the graded distribution of dl protein is initiated by selective nuclear transport. The dl protein is uniformly distributed throughout the cytoplasm of early embryos, but approximately 90 min after fertilization, dl protein present in ventral but not dorsal regions is selectively transported to the nucleus. Mutations in maternally active genes that regulate dl disrupt this transport process, resulting in an inactive, cytoplasmically localized form of the dl protein. Selective nuclear transport of dl protein was reproduced in tissue culture cells. The wild-type dl protein is largely restricted to the cytoplasm, while truncated proteins are predominantly localized within the nucleus. Transient cotransfection assays suggest that dl activates expression from several promoters in an apparently sequence-independent manner. We discuss the role of nuclear transport as a regulated process in gene expression and development.

Expression of Xenopus snail in mesoderm and prospective neural fold ectoderm

“…At least six members of the system that determines dorso-ventral pattern in Drosophila are conserved in vertebrates (Ferguson and Anderson, 1991). In Drosophila, a dorsoventral gradient of nuclear dorsal protein, established during the syncytial stage, mediates early zygotic ventral expression of two genes, snail and twist (Roth et al, 1989;Rushlow et al, 1989;Steward, 1989;Ip et al, 1992) that are required for mesoderm formation and morphogenesis (Boulay et al, 1987;Thisse et al, 1988;Leptin and Grunewald, 1990;Alberga et al, 1991;Leptin, 1991;Arora and Nusslein-Volhard, 1992). These genes are probably transcription factors as their conceptual translation products are a zinc-finger protein and an helix-loop-helix protein respectively (Boulay et al, 1987;Thisse et al, 1988).…”

Relocalization of Drosophila dorsal protein can be induced by a rise in cytoplasmic calcium concentration and the expression of constitutively active but not wild-type Toll receptors

“…Plasmids pACdlWT and pACdlS6O were a gift of C. Rushlow (Rushlow et al, 1989). Large-scale preparation of plasmids for transfection was by caesium chloride centrifugation, followed by spermine precipitation (Hoopes and McClure, 1981;Maniatis et al, 1982).…”

“…Mutations of dorsal and of ten other maternal effect genes (the dorsal group) result in a common phenotype in which the blastoderm embryo fails to elaborate the normal dorso-ventral pattern elements, with all cells developing along a pathway that is normally taken by the dorsal-most cells. The mRNA encoding dorsal protein is distributed uniformly and translated in the cytoplasm, but at the syncitial blastoderm stage of development the protein forms into a dorso-ventral nuclear gradient by a process of differential nuclear localization (Roth et al, 1989;Rushlow et al, 1989;Steward, 1989). It is now thought that such position-dependent localization of dorsal is caused by the activation at ventral positions of a uniformly distributed receptor encoded by another dorsal group gene, Toll.…”

“…Previous work has shown that full-length dorsal protein, when expressed in the Drosophila cell line Schneider 2 (SL2), is anchored in the cytoplasm (Rushlow et al, 1989) probably by interaction with cactus, which is known to be expressed in these cells. More recent work has indicated a role in signal transduction for cyclic-AMPdependent protein kinase (PKA) and for wild-type and dominant Toll receptors (Norris and Manley, 1992).…”

Actin in the Drosophila embryo: Is there a relationship to developmental cue localization?

“…During this process, the dorsal (dl) gene product redistributes from the cytoplasm to the nucleus in the ventral region (42)(43)(44) . This results in the formation of ventral structures, such as mesoderm and neurectoderm, which are lacking in dl − mutants.…”