Addition and correction: the NF-kappa B-like DNA binding activity observed in Dictyostelium nuclear extracts is due to the GBF transcription factor.

We have previously reported that a NF-kappa B transduction pathway was likely to be present in the cellular slime mold Dictyostelium discoideum. This conclusion was based on several observations, including the detection of developmentally regulated DNA binding proteins in Dictyostelium nuclear extracts that bound to bona fide kappa B sequences. We have now performed additional experiments which demonstrate that the protein responsible for this NF-kappa B-like DNA binding activity is the Dictyostelium GBF (G box regulatory element binding factor) transcription factor. This result, along with the fact that no sequence with significant similarity to components of the mammalian NF-kappa B pathway can be found in Dictyostelium genome, now almost entirely sequenced, led us to reconsider our previous conclusion on the occurrence of a NF-kappa B signal transduction pathway in Dictyostelium.

CBP1 associates with the Dictyostelium cytoskeleton and is important for normal cell aggregation under certain developmental conditions.

In cells of the eukaryotic microorganism Dictyostelium discoideum, at least eight small, four-EF-hand Ca(2+)-binding proteins of unknown function are expressed at specific times during development. One of these proteins, calcium-binding protein 1 (CBP1), first appears just prior to cell aggregation and then is present at relatively constant levels throughout development. To determine a role for CBP1 during development, the protein was used as bait in a yeast two-hybrid screen to reveal putative CBP1-interacting proteins. Two proteins identified in this screen were the actin-binding proteins, protovillin and EF-1alpha. Using an in vitro binding assay, both of these proteins were found to interact with CBP1 in the absence of Ca(2+), but the interaction of CBP1 with EF-1alpha was increased substantially by Ca(2+). CBP1 was also shown by fluorescence microscopy and by binding assays to associate with the actin cytoskeleton of Dictyostelium cells during development, and these interactions were partially Ca(2+)-dependent. cbpA-null cells grew normally, but under certain developmental conditions, cell aggregation was prolonged and irregular. This defect in aggregation appeared to be related to a general reduction in cell motility rather than to a decrease in the ability of the cells to respond to the chemoattractant cAMP. Together, these results suggest that CBP1 might function to help regulate the reorganization of the Dictyostelium actin cytoskeleton during cell aggregation.

Characterization and hetereologous expression of cDNAs encoding two novel closely related Ca(2+)-binding proteins in Dictyostelium discoideum.

During a yeast two hybrid screen of a Dictyostelium cDNA library using the Ca(2+)-binding protein CBP1 as bait, we isolated a full-length cDNA encoding a novel Ca(2+)-binding protein (termed CBP4a). The protein is composed of 162 amino acids and contains four consensus EF-hands. PCR amplification of Dictyostelium genomic DNA using primers specific for the cDNA sequence resulted in the isolation of a gene encoding a different Ca(2+)-binding protein of 162 amino acids (designated CBP4b) with 90% amino acid sequence identity to CBP4a. Southern blot analysis confirmed the presence of two closely related genes in the Dictyostelium genome. CBP4a and CBP4b mRNAs are expressed at the same stages of development as CBP1 mRNA. In addition, both novel proteins bind (45)Ca(2+) and interact with CBP1 in vitro in a Ca(2+)-dependent manner.

Evidence for the presence of an NF-kappaB signal transduction system in Dictyostelium discoideum.

The Rel/NF-kappaB family of transcription factors and regulators has so far only been described in vertebrates and arthropods, where they mediate responses to many extracellular signals. No counterparts of genes coding for such proteins have been identified in the Caenorhabditis elegans genome and no NF-kappaB activity was found in Saccharomyces cerevisiae. We describe here the presence of an NF-kappaB transduction pathway in the lower eukaryote Dictyostelium discoideum. Using antibodies raised against components of the mammalian NF-kappaB pathway, we demonstrate in Dictyostelium cells extracts the presence of proteins homologous to Rel/NF-kappaB, IkappaB and IKK components. Using gel-shift experiments in nuclear extracts of developing Dictyostelium cells, we demonstrate the presence of proteins binding to kappaB consensus oligonucleotides and to a GC-rich kappaB-like sequence, lying in the promoter of cbpA, a developmentally regulated Dictyostelium gene encoding the Ca(2+)-binding protein CBP1. Using immunofluorescence, we show specific nuclear translocation of the p65 and p50 homologues of the NF-kappaB transcription factors as vegetatively growing cells develop to the slug stage. Taken together, our results strongly indicate the presence of a complete NF-kappaB signal transduction system in Dictyostelium discoideum that could be involved in the developmental process.

Cloning and expression in Escherichia coli of a cDNA encoding a developmentally regulated Ca(2+)-binding protein from Dictyostelium discoideum.

We have cloned a full-length cDNA from Dictyostelium discoideum which encodes a new Ca(2+)-binding protein. The deduced protein (termed CBP1) is composed of 156 amino acids and contains four consensus metal-ligating loop sequences found in helix-loop-helix motifs of many Ca(2+)-binding proteins. When expressed in bacteria as a GST fusion protein, CBP1 binds Ca2+ in a 45Ca2+ overlay assay. CBP1 exhibits little amino acid sequence homology with Dictyostelium calmodulin or calfumirin-1 (CAF-1) except in the putative Ca(2+)-binding regions. Moreover, unlike calmodulin and CAF-1 expression, CBP1 mRNA is expressed preferentially during the multicellular stages of development.