TagA, a putative serine protease/ABC transporter ofDictyosteliumthat is required for cell fate determination at the onset of development

Abstract

The tag genes of Dictyostelium are predicted to encode multi-domain proteins consisting of serine protease and ATP-binding cassette transporter domains. We have identified a novel tag gene, tagA, which is involved in cell type differentiation. The tagA mRNA accumulates during the first four hours of development,whereas TagA protein accumulates between two and ten hours of development and decreases thereafter. Wild-type cells express tagA in prespore cells and mature spores, defining tagA expression as prespore specific. However, tagA mutant cells that activate the tagA promoter do not sporulate, but instead form part of the outer basal disc and lower cup of the fruiting body. tagA mutant aggregates elaborate multiple prestalk cell regions during development and produce spores asynchronously and with low viability. tagA mutants produce about twice as many prestalk cells as the wild type as judged by a prestalk cell reporter construct. When mixed with wild-type cells, tagA- cells become overrepresented in the prestalk cell population, suggesting that this phenotype is cell-autonomous. These results suggest that TagA is required for the specification of an initial population of prespore cells in which tagA is expressed. Expression profiling uncovered a delay in the transcriptional program between 2 and 6 hours, coincident with TagA expression, revealing an early function for TagA. TagA also appears to play a general role in cell fate determination since tagA mutants express a spore coat protein gene (cotB) within vacuolated cells that form part of the stalk and they express a prestalk/stalk-specific gene (ecmB)within cells that become spores. The expression of TagA at two hours of development, the observed coincident delay in the transcriptional program and the subsequent mis-expression of cell-type specific genes provide evidence for cell fate determination beginning in some cells much earlier than previously believed.

Transcriptional Transitions during Dictyostelium Spore Germination

“…Microarray profiling of gene expression is an established method for distinguishing among physiological states caused by mutations, growth conditions, and drugs (3,21). It is also applicable to the analysis of Dictyostelium growth and development (19,24,41). We therefore used this tool to characterize the process of spore germination.…”

“…Clustering of multiple genes having a common annotation is used to predict the participation of a genetic pathway in a process instead of the analysis of individual genes (45). For Dictyostelium, microarray analysis of global changes in gene expression have been used to describe physiological transitions during growth and development (41) and during dedifferentiation (23) as well as for the analysis of mutant cell physiology (19,22,24,25). It has also been shown that combining cluster analysis and gene annotation can predict gene function.…”

Tissue-specific G1-phase cell-cycle arrest prior to terminal differentiation inDictyostelium

“…It will be important to determine whether the late mitotic events and G1-phase arrest are required for spore cell differentiation or stalk formation. Certainly, prespore-specific genes and genes that become prespore enriched are expressed in some cells well before mitosis begins so it is unlikely that G1 arrest is required for all aspects of prespore differentiation (Fosnaugh and Loomis, 1993;Good et al, 2003;Haberstroh and Firtel, 1990;Iranfar et al, 2001;VanDriessche et al, 2002;Williams et al, 1989). Microtubule destabilizing drugs that block mitosis do not block development suggesting that prespore cells need not divide prior to spore differentiation (Cappuccinelli and Ashworth, 1976;Cappuccinelli et al, 1979).…”

A secreted factor represses cell proliferation inDictyostelium

“…All procedures were at room temperature. 0.8% v/v Nonidet P-40 alternative (2 ml) (Calbiochem, La Jolla, CA) in PB was then added to the tube (Good et al, 2003). The tube was rocked gently for 10 minutes and the filter was then removed.…”