Phosphorylation of spore coat proteins during development of Dictyostelium discoideum

Multiple O-glycoforms on the spore coat protein SP96 in Dictyostelium discoideum. Fuc(alpha1-3)GlcNAc-alpha-1-P-Ser is the major modification

A decreased level of fucosylation on certain spore coat proteins of Dictyostelium discoideum alters the permeability of the spore coat. Here the post-translational modifications of a major spore coat protein, SP96, are studied in a wild type strain (X22) and a fucosylation-defective mutant (HU2470). A novel phosphoglycan structure on SP96 of the wild type strain, consisting of Fuc(alpha1-3)GlcNAc-alpha-1-P-Ser(,) was identified by electrospray ionization mass spectrometry and NMR. It was shown using monosaccharide and gas chromatography mass spectrometry analysis that SP96 in the mutant HU2470 contained approximately 20% of wild type levels of fucose, as a result of a missing terminal fucose on the novel glycan structure. The results support previous predictions, based on inhibition studies on different fucose-deficient strains, about the nature of monoclonal antibody epitopes identified by monoclonal antibodies MUD62 and MUD166, which are known to identify O-linked glycans (Champion, A., Griffiths, K., Gooley, A. A., Gonzalez, B. Y., Gritzali, M., West, C. M., and Williams, K. L. (1995) Microbiology 141, 785-797). Quantitative studies on wild type SP96 indicated that there were approximately 60 sites with phosphodiester-linked N-acetylglucosamine-fucose disaccharide units and a further approximately 20 sites with fucose directly linked to the protein. Over 70% of the serine sites are modified, with less than 1% of these sites as phosphoserine. Threonine and tyrosine residues were not found to be modified.

The PsB glycoprotein complex is secreted as a preassembled precursor of the spore coat in Dictyostelium discoideum

The PsB glycoprotein in Dictyostelium discoideum is one of a diverse group of developmentally regulated, prespore-cell-specific proteins, that contain a common O-linked oligosaccharide. This post-translational modification is dependent on the wild-type modB allele. The PsB protein exists as part of a multiprotein complex of six different proteins, which have different post-translational modifications and are held together by both covalent and non-covalent interactions (Watson et al. (1993). J. Biol. Chem. 268, 22634–22641). In this study we have used microscopic and biochemical analyses to examine the cellular localization and function of the PsB complex during development. We found that the PsB complex first accumulates in prespore vesicles in slug cells and is secreted later during culmination and becomes localized to both the extracellular matrix of the apical spore mass of mature fruiting bodies and to the inner layer of the spore coat. The PsB associated with the spore coat is covalently bound by disulfide bridges. The PsB protein always exists in a multiprotein complex, but the composition of the PsB complex changes during secretion and spore maturation. Some of the PsB complex proteins have been identified as spore coat proteins. These data demonstrate that some of the proteins that form the spore coat exist as a preassembled precursor complex. The PsB complex is secreted in a developmentally regulated manner during the process of spore differentiation, at which time proteins of the complex, as well as additional spore coat proteins, become covalently associated in at least two forms of extracellular matrix: the interspore matrix and the spore coat. These and other studies show that proteins with modB dependent O-linked oligosaccharides are involved in a wide variety of processes underlying morphogenesis in this organism. These developmental processes are the direct result of cellular mechanisms regulating protein targeting, assembly and secretion, and the assembly of specific extracellular matrices.

Developmental expression and characterization of the gene encoding spore coat protein SP60 in Dictyostelium discoideum

The complete coding sequence, upstream sequence and developmental expression of Dictyostelium discoideum AX2 spore coat protein gene SP60 is reported. The gene contains two exons, 154bp and 1121bp long, separated by a 119bp intron, and encodes a protein of 46,925 molecular weight plus a 23-amino-acid hydrophobic leader sequence. The N-terminus of the mature protein consists of four copies of a perfect hexapeptide repeat (GDWNNN). The central region is rich in cysteine residues, including four highly conserved cysteine-rich repeats with homology to 'EGF-like' repeats. The C-terminus is aspartate-rich and composed of multiple imperfect copies of a D(G/D)DYD repeat followed by several repeats of the tetrapeptide DNDW and derived sequences. A TATA box promoter motif juxtaposed to an oligo(dA) stretch lies 52bp upstream of the main transcriptional start site of the gene. Six AC-rich boxes occur in the region -327 to -556, all of which contain the consensus sequence CACAC. Two GC-rich boxes and a C-rich element (TTACCCCA) are also present upstream. Another open reading frame is positioned a short distance downstream of the SP60 gene in the opposite transcriptional orientation. Expression of the SP60 gene ceases upon disaggregation to single cells and cannot be restored by high levels of extracellular cAMP either alone or in combination with conditioned medium factors.

Major changes in phosphorylation of chromatin-associated non-histone proteins accompany development in the slime mould Dictyostelium discoideum

Striking changes in protein and RNA synthesis were shown to accompany development in the slime mould Dictyostelium discoideum. These changes are, at least in part, directly attributable to control at the level of transcription. Analysis of nuclear proteins and their states of phosphorylation by two-dimensional gel electrophoresis and autoradiography showed only minor changes in the species of proteins detectable by Coomassie Blue staining during the vegetative growth and development phases. Major changes, however, were detected in their patterns of phosphorylation, with major differences from the vegetative pattern occurring during both early development (0-2h) and late development (8-12h). These changes coincide with major changes in polypeptide synthesis and in nuclear and cytoplasmic RNA complexity.

Spore coat protein synthesis during development of Dictyostelium discoideum requires a low-molecular-weight inducer and continued multicellularity

The major spore coat proteins of Dictyostelium discoideum are synthesized during the culmination stage of development. In an attempt to examine the regulatory mechanisms involved, spore coat protein synthesis by pseudoplasmodia harvested prior to culmination and incubated in submerged culture under various environmental conditions has been monitored. It is reported that the synthesis of spore coat proteins SP170, SP103, SP94, SP82, SP76, and SP55 is dependent upon the presence of a low-molecular-weight (Mr approx 100), heat-stable factor secreted by cells incubated at high density in buffer. Previous studies have implicated cyclic AMP, ammonia, and amino acids in spore cell differentiation. Partial purification of the spore coat protein inducing factor (SPIF), together with attempts to mimic its activity, indicate that SPIF is not identical with any of these molecules and it is probably also distinct from DIF and "fruit juice," two other factors which regulate the spore-stalk decision and the initiation of culmination, respectively, in D. discoideum. In addition to SPIF, the continued expression of the spore coat protein genes also requires that the integrity of the pseudoplasmodium be maintained. Unlike the expression of many other genes after aggregation, this latter requirement cannot be replaced by exogenous cyclic AMP. Termination of spore coat protein gene expression occurs despite the presence of excess exogenous SPIF and hence involves mechanisms other than the destruction or depletion of SPIF.

Synthesis of spore proteins during development of Dictyostelium discoideum

The pattern of synthesis of the spore coat proteins during development of Dictyostelium discoideum has been determined by using immunoprecipitation with spore protein antibody. SP170, SP103, 'SP94', SP82, SP76 and SP55 are all first synthesized just prior to the 'Mexican hat' stage of development (16-18h), but the synthesis of SP72 is delayed. This protein is apparently synthesized as a precursor, P66, which is modified during spore maturation to yield SP72. The nature of the modification is unknown. At their peak period of synthesis during early culmination (18-20h), the spore coat proteins account for 5-9% of total protein synthesis. Shortly after synthesis, these proteins are inserted into the spore coat, where all except SP103 become disulphide-cross-linked during the period 24-30h. SP3 does not accumulate until disulphide-cross-linking of the major spore coat proteins occurs and is itself disulphide-cross-linked into the spore coat. Several additional proteins that are accumulated during development have also been identified, namely P31, P25, P21 and P18. P25 first appears at 18-20h and then continues to be made throughout development. P31 synthesis begins at 12-14h and then largely ceases after approx. 20 h of development. The genes for both P21 and P18 are first expressed early in development, starting at 9-12h. P21 synthesis ceases at approx. 14h, but P18 continues to be synthesized throughout the rest of development. The marked differences in the time period of accumulation of these proteins compared with the co-ordinated syntheses of SP170, SP103, 'SP94', SP82, SP76 and SP55 provide a useful system for analysis of the mechanism of temporal gene expression during development.