LAGC IS REQUIRED FOR CELL-CELL INTERACTIONS THAT ARE ESSENTIAL FOR CELL-TYPE DIFFERENTIATION IN DICTYOSTELIUM

Strain AK127 is a developmental mutant of Dictyostelium discoideum tha t was isolated by restriction enzyme-mediated integration (REMI). Muta nt cells aggregate normally but are unable to proceed past the loose a ggregate stage. The cloned gene, lagC (loose aggregate C), encodes a n ovel protein of 98 kD that contains an amino-terminal signal sequence and a putative carboxy-terminal transmembrane domain. The mutant strai n AK127 shows no detectable lagC transcript upon Northern analysis, in dicating that the observed phenotype is that of a null allele. Express ion of the lagC cDNA in AK127 cells complements the arrest at the loos e aggregate stage, indicating that the mutant phenotype results from d isruption of the lagC gene. In wild-type cells, lagC mRNA is induced a t the loose aggregate stage and is expressed through the remainder of development. lagC(-) null cells aggregate but then disaggregate and re aggregate to form small granular mounds. Mature spores are produced at an extremely low efficiency (<0.1% of wild type), appearing only afte r similar to 72 hr, whereas wild-type strains produce mature spores by 26 hr. lagC(-) null cells accumulate reduced levels of transcripts fo r the prestalk-enriched genes rasD and CP2 and do not express the DIE- induced prestalk-specific gene ecmA or the cAMP-induced prespore-speci fic gene SP60 to significant levels. In chimeric organisms resulting f rom the coaggregation of lagC(-) null and wild-type cells, cell-type-s pecific gene expression is rescued in the lagC(-) null cells; however, lagC(-) prespore cells are localized to the posterior of the prespore region and do not form mature spores, suggesting that LagC protein ha s both no cell-autonomous and cell-autonomous functions. Overexpressio n of lagC from an actin promoter in both wild-type and lagC(-) cells c auses a delay at the tight aggregate stage, the first stage requiring LagC activity. These results suggest that the LagC protein functions a s a nondiffusible cell-cell signaling molecule that is required for mu lticellular development.