Formation of the Dictyostelium spore coat

Abstract

The spore coat forms as a rigid extracellular wall around each spore cell during culmination. Coats purified from germinated spores contain multiple protein species and an approximately equal mass of polysaccharide, consisting mostly of cellulose and a galactose/N-acetylgalactosamine polysaccharide (GPS). All but the cellulose are prepackaged during prespore cell differentiation in a regulated secretory compartment, the prespore vesicle. The morphology of this compartment resembles an anastomosing, tubular network rather than a spherical vesicle. The molecules of the prespore vesicles are not uniformly mixed but are segregated into partially overlapping domains. Although lysosomal enzymes have been found in the prespore vesicle, this compartment does not function as a lysosome because it is not acidic, and a common antigen associated with acid hydrolases is found in another, acidic vesicle population. All the prespore vesicle profiles disappear at the time of appearance of their contents outside of the cell; this constitutes an early stage in spore coat formation, which can be detected both by microscopy and flow cytometry. As an electron-dense layer, the future outer layer of the coat, condenses, cellulose can be found and is located immediately beneath this outer layer. Certain proteins and the GPS become associated with either the outer or inner layers surrounding this middle cellulose layer. Assembly of the inner and outer layers occurs in part from a pool of glycoproteins that is shared between spores, and unincorporated molecules loosely reside in the interspore matrix, a location from which they can be easily washed away. When the glycosylation of several major protein species is disrupted by mutation, the coat is assembled, but differences are found in its porosity and the extractibility of certain proteins. In addition, the retention or loss of proteolytic fragments in the mutants indicates regions of spore coat proteins that are required for association with the coat. Comparative examination of the macrocyst demonstrates that patterns of molecular distributions are not conserved between the macrocyst and spore coats. Thus spore coat assembly is characterized by highly specific intermolecular interactions, leading to saturable associations of individual glycoproteins with specific layers and the exclusion of excess copies to the interspore space.

Dictyostelium Hip1r contributes to spore shape and requires epsin for phosphorylation and localization

“…The coat that encloses a dormant amoeba within the spore comprises cellulose and glycoproteins (West and Erdos, 1990). Most of these glycoproteins are delivered to the exterior of pre-spore cells when PSVs, containing pre-assembled protein complexes, fuse with the plasma membrane (Srinivasan et al, 1999;Srinivasan et al, 2000a).…”

Disruption of the sporulation-specific gene spiA in Dictyostelium discoideum leads to spore instability.

“…It is associated in a detergent-resistant manner with the inner face of spore coats such that it can be reached by specific antibodies only when they are cracked or broken. The spore coats of mutant strains appear normal, with electron-dense inner and outer layers separated by a cellulosic middle layer, just as seen in the spore coats of the wild-type strains (Hohl and Hamamoto 1969;West and Erdos 1990). Mutations in the wetA gene of the filamentous fungus Aspergillis nigans also result in water-sensitive spores, but in this case the spores are missing two layers of the coat (Sewall et al 1990).…”

“…To sporulate, the prespore cells dehydrate and the prespore vesicles fuse with the plasma membrane to release the spore coat components (Hohl and Hamamoto 1969). These components assemble to form the inner and outer layers of the coat; a middle cellulose layer is synthesized and deposited between the two proteinaceous layers (West and Erdos 1990). This process results in spores that are resistant to heat and desiccation (Cotter and Raper 1968).…”

Outside-In Signaling of Cellulose Synthesis by a Spore Coat Protein in Dictyostelium

“…In Dictyostelium, the spore coat is formed de novo at the surface of prespore cells at the end of the fruiting body developmental cycle, about 24 h after starvation (34). The spore coat is a relatively simple cell wall consisting of about 50% cellulose and 50% protein, and 2% is a galuran polysaccharide containing Gal, GalNAc, and possibly GalUA (39).…”