SunB, a novel Sad1 and UNC-84 domain-containing protein required for development of Dictyostelium discoideum.

A gene, sunB, encoding a novel class of Sad1 and UNC-84 (SUN) domain, was isolated from a cDNA screen for suppressors of a mutation in Dd-STATa - a Dictyostelium homologue of metazoan STAT (signal transducers and activators of transcription). The SunB protein localized in the area around the nucleus in growing cells, but in the multicellular stages it was predominantly found in prespore vacuoles (PSVs). A disruptant of sunB was multinucleated in the vegetative phase; during development it formed mounds with multiple tips and failed to culminate. The mutation was cell autonomous, and showed reduced expression of the prespore marker gene pspA and elevated expression of marker genes for prestalk AB cells. Interestingly, the level of SunB was abnormally high in the prestalk cells of Dd-STATa mutants, which are defective in culmination. We conclude that SunB is essential for accurate prestalk/prespore differentiation during Dictyostelium development and that its cell-type dependent localization is regulated by a Dd-STATa-mediated signaling pathway.