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Disruption of the gene encoding the cell adhesion molecule DdCAD-1 leads to aberrant cell sorting and cell-type proportioning during Dictyostelium development

¹ Banting and Best Department of Medical Research and Department of Biochemistry, University of Toronto, Toronto, Ontario M5G 1L6, Canada
² Center for Molecular Genetics, Department of Biology, University of California at San Diego, La Jolla, CA 92037, USA

*Author for correspondence (e-mail: chi.hung.siu{at}utoronto.ca)

Accepted 16 May 2002

The cadA gene in Dictyostelium encodes the Ca²⁺-dependent cell adhesion molecule DdCAD-1, which is expressed soon after the initiation of development. To investigate the biological role of DdCAD-1, the cadA gene was disrupted by homologous recombination. The cadA-null cells showed a 50% reduction in EDTA-sensitive cell adhesion. The remaining EDTA-sensitive adhesion sites were resistant to dissociation by anti-DdCAD-1 antibody, suggesting that they were distinct adhesion sites. Cells that lacked DdCAD-1 were able to complete development and form fruiting bodies. However, they displayed abnormal slug morphology and culmination was delayed by 6 hours. The yield of spores was reduced by 50%. The proportion of prestalk cells in cadA^– slugs showed a 2.5-fold increase over the parental strain. When cadA^– cells were transfected with pcotB::GFP to label prespore cells, aberrant cell-sorting patterns in slugs became apparent. When mutant prestalk cells were mixed with wild-type prespore cells, mutant prestalk cells were unable to return to the anterior position of chimeric slugs, suggesting defects in the sorting mechanism. The wild-type phenotype was restored when cadA^– cells were transfected with a cadA-expression vector. These results indicate that, in addition to cell-cell adhesion, DdCAD-1 plays a role in cell type proportioning and pattern formation.

Key words: Cell-cell adhesion, Gene disruption, Cell-type differentiation, Pattern formation, Dictyostelium