Rates of accumulation of glycosidase activities during growth and differentiation of Dictyostelium discoideum

Differential developmental expression of the rep B and rep D xeroderma pigmentosum related DNA helicase genes from Dictyostelium discoideum

DNA helicases are essential to many cellular processes including recombination, replication and transcription, and some helicases function in multiple processes. The helicases encoded by the Xeroderma pigmentosum (XP) B and D genes function in both nucleotide excision repair and transcription initiation. Mutations that affect the repair function of these proteins result in XP while mutations affecting transcription result in neurological and developmental abnormalities, although the underlying molecular and cellular basis for these phenotypes is not well understood. To better understand the developmental roles of these genes, we have now identified and characterized the rep B and rep D genes from the cellular slime mold Dictyostelium discoideum . Both genes encode DNA helicases of the SF2 superfamily of helicases. The rep D gene contains no introns and the rep B gene contains only one intron, which makes their genomic structures dramatically different from the corresponding genes in mammals and fish. However the predicted Dictyostelium proteins share high homology with the human XPB and XPD proteins. The single copy of the rep B and D genes map to chromosomes 3 and 1, respectively. The expression of rep B and D (and the previously isolated rep E) genes during multicellular development was examined, and it was determined that each rep gene has a unique pattern of expression, consistent with the idea that they have specific roles in development. The pattern and extent of expression of these genes was not affected by the growth history of the cells, implying that the expression of these genes is tightly regulated by the developmental program. The expression of the rep genes is a very early step in development and may well represent a key event in the initiation of development in this organism.

A locus regulating N-acetylglucosaminidase synthesis during development in dictyostelium

The cellular specific activity of N-acetylglucosaminidase increases during development in Dictyostelium discoideum. A monoclonal antibody which specifically recognizes Mr 68,000 and 67,000 forms of N-acetylglucosaminidase was used to show that changes in the relative rate of enzyme synthesis during development parallel the pattern of enzyme accumulation. Developmental and regulatory mutants were isolated to study the relationship between development and enzyme accumulation. No evidence was obtained for any dependence of enzyme accumulation on those genes that are required for aggregation. However, a separate regulatory locus was identified which is involved in enzyme accumulation. Mutations in this gene, nagC, prevent enzyme accumulation during development by preventing an increase in the relative synthetic rate of N-acetylglucosaminidase. The accumulation of other enzymes is unaffected and the mutation causes no developmental defects other than those caused by the loss of N-acetylglucosaminidase activity. The nagC mutation, which is recessive, maps to linkage group VI and is therefore unlinked to the structural gene for N-acetylglucosaminidase.

The regulation of 'early' enzymes during the development and dedifferentiation of Dictyostelium discoideum

The specific activities of the enzymes alpha-mannosidase and N-acetylglucosaminidase increase immediately after the initiation of the development of bacterially grown cell cultures of Dictyostelium discoideum. The regulation of these two enzymes was found to be dissociable in the developmental timer mutant, FM-1, which aggregates 4.5 h earlier than wild-type cells due to the absence of the first rate-limiting component of the preaggregative period. The increase in alpha-mannosidase activity occurs in the absence of the first rate-limiting component, but the increase in N-acetylglucosaminidase activity does not. These results indicate the following: (1) the increase in the specific activity of alpha-mannosidase is not related to the timing of subsequent developmental stages; (2) the increase in the specific activity of N-acetylglucosaminidase is not necessary for the subsequent developmental program; and (3) either the increase in the specific activity of N-acetylglucosaminidase is dependent upon progress through the first rate-limiting component, or the increase in this enzyme activity and the first rate-limiting component are both dependent upon an early event for which FM-1 is defective. In addition to early development, we monitored the two enzyme activities during dedifferentiation. The results demonstrate that there is no difference between dedifferentiating wild-type cells and dedifferentiation-defective mutant HI-4 cells. Changes in enzyme specific activity accompanying dedifferentiation are dependent upon the composition of the dedifferentiation-inducing media and are consistent with the levels of these enzymes observed in cells growing in the different nutrient media.

Developmental regulation of nuclear glycosyl transfer in Cictyostelium discoideum

Glycosyltransferases are active in isolated cell nuclei of Dictyostelium discoideum. Since the biological role of these nuclear enzymes is unknown we assayed their activity in nuclear fractions from vegetatively growing and differentiating amoebae. The nuclear N-acetylglucosaminyl transfer is highest in the vegetative stage and shortly after induction of differentiation, then declines gradually. This result is discussed with respect to a possible role for glycoproteins and mucopolysaccharides in the structure or in the transcriptional and replicative functions of the genome.

Properties and developmental regulation of an alpha-d-galactosidase from Dictyostelium discoideum

An alpha-D-galactosidase was detected in cells of the cellular slime mould, Dictyostelium discoideum, at all stages of development. Its specific activity was highest during early development (interphase), and this accumulation of enzyme appears to require protein synthesis de novo. Its subcellular distribution differs from that of other D. discoideum glycosidases, since most activity was recovered in the soluble fraction. No evidence was obtained for more than one isoenzymic form after subjection of extracts to electrophoresis and various chromatographic procedures. It is excreted from the cell during development, but no evidence was found for an extracellular function for the enzyme.

Rates of degradation and synthesis of glycosidases de novo during growth and differentiation of Dictyostelium discoideum

1. Injection of a purified preparation of beta-N-acetylglucosaminidase from the spent growth medium of myxamoebae of Dictyostelium discoideum into rabbits gave rise to an antibody preparation containing both anti-alpha-glucosidase and anti-beta-acetylglucosaminidase activities. 2. These two activities were shown to reside in different immunoglobulin molecules and it was concluded that the beta-N-acetylglucosaminidase preparation contained trace amounts of highly antigenic alpha-glucosidase. 3. A single precipitin band having beta-N-acetylglucosaminidase activity was formed in Ouchterlony plates when this antibody preparation was tested against extracts obtained from differentiated cells or from myxamoebae grown either axenically or on bacteria. 4. The antibody preparation was used to show that both beta-N-acetylglucosaminidase and alpha-glucosidase molecules are synthesized de novo from isotopically labelled amino acids during both the growth and differentiation phases of the life cycle and to show that neither of these proteins is significantly degraded during the growth phase or during the first 9h of differentiation. 5. The rates of accumulation of these assayable enzyme activities are thus equal to their rates of synthesis during growth and early differentiation. 6. The factors regulating cellular enzyme activity during the life cycle of D. discoideum are discussed.