A DICTYOSTELIUM MUTANT WITH DEFECTIVE AGGREGATE SIZE DETERMINATION

Starved Dictyostelium cells aggregate into groups of roughly 10(5) cel ls, We have identified a gene which, when repressed by antisense trans formation or homologous recombination, causes starved cells to form la rge numbers of small aggregates. We call the gene smlA for small aggre gates, A roughly 1.0 kb smlA mRNA is expressed in vegetative and early developing cells, and the mRNA level then decreases at about 10 hours of development, The sequence of the cDNA and the derived amino acid s equence of the SmlA protein show no significant similarity to any know n sequence. There are no obvious motifs in the protein or large region s of hydrophobicity or charge, Immunoflur orescence and staining of We stern blots of cell fractions indicates that SmlA is a 35x10(3) M(r) c ytosolic protein present in all vegetative and developing cells and is absent from smlA cells, The absence of SmlA does not affect the growt h rate, cell cycle, motility, differentiation, or developmental speed of cells, Synergy experiments indicate that mixing 5% smlA cells with wild-type cells will cause the wild-type cells to form smaller fruitin g bodies and aggregates, Although there is no detectable SmlA protein secreted from cells, starvation medium conditioned by smlA cells will cause wild-type cells to form large numbers of small aggregates, The c omponent in the smlA-conditioned media that affects aggregate size is a molecule with a molecular mass greater than 100x10(3) M(r) that is n ot conditioned media factor, phosphodiesterase or the phosphodiesteras e inhibitor, The data thus suggest that the cytosolic protein SmlA reg ulates the secretion or processing of a secreted factor that regulates aggregate size.