The effects of inhibition of development in Dictyostelium discoideum on changes in plasma membrane composition and topography

Specific cell-cell contacts are essential for induction of gene expression during differentiation of Dictyostelium discoideum

Postaggregation Dictyostelium discoideum cells contain 2000-3000 mRNA species that are absent from pre-aggregation cells. These aggregation-dependent sequences compose 30% of the mass of the late mRNA and represent the transcription products of an additional 11% of the single-copy genome. By analysis of mutants that are blocked at different stages of differentiation, we show that induction of expression of these genes is correlated with the formation of tight cell-cell contacts that resist EDTA. In particular, mutants that exhibit chemotaxis and aggregate to form loose mounds but do not form cell-cell contacts that resist EDTA fail to induce these late mRNA and protein species. By contrast, mutants that form normal contacts but progress no further through development do express the late mRNA species. Thus, interactions at the cell surface are involved in developmental induction of a large group of coregulated mRNAs. We have employed two independent assays for these developmentally regulated mRNAs: hybridization of gel-separated RNAs to cloned nuclear DNAs and hybridization of mRNA to a cDNA probe specific for the population of 2000-3000 regulated sequences.

Developmental changes in protein composition and the actin-binding protein ponticulin in Dictyostelium discoideum plasma membranes purified by an improved method

We have used a new combination of previously-described methods to obtain a 29-fold purification of plasma membranes from Dictyostelium discoideum. In this procedure, the pellet from a cell lysate is centrifuged through a high-pH sucrose gradient and then through a Renografin gradient. Electron microscopy shows that the resultant "Renografin membranes" are essentially homogeneous. As measured by enzymatic marker assays, contamination with mitochondria, lysosomes, and endoplasmic reticulum is minimal. As assayed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), the protein composition of Renografin membranes is similar to that of highly purified membranes isolated using concanavalin A stabilization and detergent extraction. Using Renografin membranes, we have examined developmental changes in the membrane protein composition. In agreement with previous investigations, we observe major changes in lectin-binding glycoproteins and cell-surface-labeled proteins during the first 18 h of D. discoideum development. In contrast to most previous work, which may have employed plasma membranes of lesser purity, we also observe major changes in silver-stained membrane proteins. We conclude that many developmentally regulated proteins, previously thought to be minor membrane constituents, are a larger proportion of the plasma membrane than originally believed. The observed changes in membrane protein composition may correlate with changes in plasma membrane functions during development. For instance, ponticulin, the major salt-sensitive F-actin-binding protein in plasma membranes from vegetative cells, increases at least twofold in plasma membranes during early development when the cells are chemotaxing into large aggregates. The amount of plasma membrane ponticulin then decreases during the pseudoplasmodial stage.

Purification of membranes and identification of phase-specific proteins of the dimorphic pathogenic fungus Histoplasma capsulatum

Plasma membrane vesicles from the yeast and mycelial phases of Histoplasma capsulatum have been purified and characterized. The method of purification involved differential centrifugation of ballistically fractured cells followed by sedimentation through discontinuous sucrose density gradient and equilibrium centrifugation. Purity of the preparation was assessed by electron microscopy. The protein composition of the membrane preparations from the yeast and mycelial phases of the fungus was analyzed by polyacrylamide gels. A comparison of the two morphologic phases revealed quantitative and qualitative differences in the expressions of several membrane-specific proteins. Physical differences in the appearance of the membranes were also observed by electron micrography of membrane preparations. Alteration in membrane fluidity may be one of the many causes for differences in the appearance of membrane vesicles in the two phases.