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 Ca2+-binding proteins of unknown function are ex pressed at specific times during development. One of these proteins, calciu m-binding protein 1 (CBP1), first appears just prior to cell aggregation an d 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, pro tovillin and EF-1 alpha. Using an in vitro binding assay, both of these pro teins were found to interact with CBP1 in the absence of Ca2+, but the inte raction of CBP1 with EF-1 alpha was increased substantially by Ca2+. CBP1 w as 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 Ca2+-dependent. cbpA-null cells grew nor mally, but under certain developmental conditions, cell aggregation was pro longed and irregular, This defect in aggregation appeared to be related to a general reduction in cell motility rather than to a decrease in the abili ty of the cells to respond to the chemoattractant cAMP. Together, these res ults suggest that CBP1 might function to help regulate the reorganization o f the Dictyostelium actin cytoskeleton during cell aggregation. (C) 2000 Ac ademic Press.