A Retinoblastoma Orthologue Is Required for the Sensing of a Chalone in Dictyostelium discoideum

Abstract

ABSTRACT Retinoblastoma-like proteins regulate cell differentiation and inhibit cell proliferation. The Dictyostelium discoideum retinoblastoma orthologue RblA affects the differentiation of cells during multicellular development, but it is unclear whether RblA has a significant effect on Dictyostelium cell proliferation, which is inhibited by the secreted proteins AprA and CfaD. We found that rblA − cells in shaking culture proliferate to a higher density, die faster after reaching stationary density, and, after starvation, have a lower spore viability than wild-type cells, possibly because in shaking culture, rblA − cells have both increased cytokinesis and lower extracellular accumulation of CfaD. However, rblA − cells have abnormally slow proliferation on bacterial lawns. Recombinant AprA inhibits the proliferation of wild-type cells but not that of rblA − cells, whereas CfaD inhibits the proliferation of both wild-type cells and rblA − cells. Similar to aprA − cells, rblA − cells have a normal mass and protein accumulation rate on a per-nucleus basis, indicating that RblA affects cell proliferation but not cell growth. AprA also functions as a chemorepellent, and RblA is required for proper AprA chemorepellent activity despite the fact that RblA does not affect cell speed. Together, our data indicate that an autocrine proliferation-inhibiting factor acts through RblA to regulate cell density in Dictyostelium , suggesting that such factors may signal through retinoblastoma-like proteins to control the sizes of structures such as developing organs or tumors.