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Separable and redundant regulatory determinants in Cactus mediate its dorsal group dependent degradation

Nielsen Q. Fernandez¹, Jörg Grosshans^2,*, Jason S. Goltz^1,3 and David Stein^1,3,

¹ Department of Molecular Genetics, Albert Einstein College of Medicine, Bronx, NY 10461, USA
² Max-Planck-Institut fur Entwicklungsbiologie, Tubingen, Germany
³ Section of Molecular Cell and Developmental Biology, Institute for Cellular and Molecular Biology, University of Texas, Austin, TX 78712, USA
^* Present address: Department of Molecular Biology, Princeton University, Princeton, NJ 08544, USA

Author for correspondence (e-mail: d.stein{at}mail.utexas.edu)

Accepted May 9, 2001

Dorsal-ventral polarity within the Drosophila syncytial blastoderm embryo is determined by the maternally encoded dorsal group signal transduction pathway that regulates nuclear localization of the transcription factor Dorsal. Nuclear uptake of Dorsal, a Rel/NFB homolog, is controlled by the interaction with its cognate IB inhibitor protein Cactus, which is degraded on the ventral side of the embryo in response to dorsal group signaling. Previous studies have suggested that an N-terminally located kinase target motif similar to that found in IB proteins is involved in the spatially controlled degradation of Cactus. We report studies of the in vivo function and distribution of fusion proteins comprising segments of Cactus attached to Escherichia coli ß-galactosidase (lacZ). Full-length Cactus-lacZ expressed in vivo normalizes the ventralized phenotype of embryos that lack Cactus and faithfully reconstitutes dorsal group-regulated degradation, while fusion protein constructs that lack the first 125 amino acids of Cactus escape dorsal group-dependent degradation. Furthermore, Cactus-lacZ constructs that lack only the putative IB-dependent kinase target-like motif can nevertheless undergo spatially regulated dorsal group-dependent degradation and we have identified the regulatory determinant responsible for dorsal group-dependent degradation of Cactus in the absence of this motif. Taken together, our studies indicate the presence of two distinct redundantly acting determinants in the N terminus of Cactus that direct dorsal group-dependent degradation. Strikingly, the regulatory domain of human IB can also direct polarized degradation of Cactus-lacZ fusion protein.

Key words: Drosophila, Dorsal-ventral polarity, Dorsal group, Dorsal protein, Cactus, IB, IKK, NFB, protein degradation

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