The membrane-bound alkaline phosphatase and 5'-nucleotidase activities of vegetative cells of Dictyostelium discoideum

Analysis of 5' nucleotidase and alkaline phosphatase by gene disruption in Dictyostelium

In Dictyostelium discoideum a phosphatase with a high pH optimum is known to increase in activity during cell differentiation and become localized to a narrow band of cells at the interface of prespore and prestalk cells. However, it was not clear if this activity is due to a classical "alkaline phosphatase" with broad range substrate specificity or to a "5'nucleotidase" with high substrate preference for 5'AMP. We attempted to disrupt the genes encoding these two phosphatase activities in order to determine if the activity that is localized to the interface region resides in either of these two proteins. During aggregation of 5nt null mutants, multiple tips formed rather than the normal single tip for each aggregate. In situ phosphatase activity assays showed that the wt and the 5nt gene disruption clones had normal phosphatase activity in the area between prestalk and prespore cell types, while the alp null mutants did not have activity in this cellular region. Thus, the phosphatase activity that becomes localized to the interface of the prestalk and prespore cells is alkaline phosphatase.

Purification and renaturation of Dictyostelium recombinant alkaline phosphatase by continuous elution electrophoresis

A 1583 bp fragment of Dictyostelium alp cDNA (94% of the gene) was cloned in pET32a+. The enzyme was expressed in an inactive form in the inclusion body of the expression host BL21-CodonPlus (DE3)-RIL. The recombinant ALP constituted more than 50% of the total protein in the inclusion body and 25-30% of the total protein in the expression host after 3 h induction with IPTG at 37 degrees C. A continuous elution polyacrylamide gel electrophoresis procedure was used to purify the recombinant enzyme. This technique yielded a homogeneous protein that retained enzymatic activity after dialysis without further treatment. A yield of 5mg per liter of culture broth was obtained with a specific activity of approximately 0.7 nmol/min/mg protein (0.7 mU/mg). Immunoinhibition studies using a polyclonal antibody produced against the recombinant protein showed complete inhibition of enzymatic activity when the enzyme was preincubated with the antibody at a 1:1000 dilution. The enzyme exhibited a pH optimum of approximately 9.0. The substrate specificity indicated that the Dictyostelium enzyme is a typical broad range alkaline phosphatase.

5'-Nucleotidase in Dictyostelium: protein purification, cloning, and developmental expression

5'-Nucleotidase (5NU) in Dictyostelium discoideum is an enzyme that shows high substrate specificity to 5'-AMP. The enzyme has received considerable attention in the past because of the critical role played by cyclic AMP in cell differentiation in this organism. Degradation of cAMP by cAMP phosphodiesterase (PDE) produces 5'-AMP, the substrate of 5NU. During the time course of development, the enzyme activity of 5NU increases and becomes restricted to a narrow band of cells that form the interface between the prestalk/prespore zones. We have purified a polypeptide associated with 5NU enzyme activity. Protein sequence of this peptide was obtained from mass spectrometry and Edman degradation. Polymerase chain reaction PCR amplification of genomic DNA using degenerate oligonucleotides and a search of sequences of a cDNA project yielded DNA fragments with sequence corresponding to the peptide sequence of 5NU. In addition, a clone was found that corresponded to the classical 'alkaline phosphatase' (AP) as described in several organisms. The sequences of the 5NU and AP cDNAs were not similar, indicating they are the products of separate genes and that both genes exist in Dictyostelium. Analysis of the expression of 5nu during Dictyostelium development by Northern blotting determined that the gene is developmentally regulated. Southern blot analysis showed a single form of the 5nu gene. Targeted gene disruption and knockout mutagenesis using the 5nu sequences suggested that a 5nu mutation may be lethal.

Studies on the mechanism of action of the alkaline phosphatase from Dictyostelium discoideum

The Dictyostelium discoideum alkaline phosphatase was investigated kinetically in an attempt to elucidate its mechanism of action. Analysis of the hydrolysis of p-nitrophenyl phosphate by stopped-flow spectrophotometry revealed biphasic kinetics, suggesting a double displacement enzyme mechanism. Furthermore, Tris stimulated activity in an uncompetitive manner, a result that was consistent with this interpretation. The enzyme was inhibited reversibly by phosphate at low ionic strength, but the inhibition was irreversible at high ionic strength and the latter effect was enhanced at alkaline pH values. These results indicate that high ionic strength and alkaline pH conditions bring about a conformational change that renders the enzyme susceptible to irreversible inhibition by phosphate.

Purification of plasma membrane from Acanthamoeba castellanii

A simple method for isolation of plasma membrane from Acanthamoeba using self-generating gradients of Percoll is described. To obtain a membrane marker, intact amoebae were radioiodinated and the distribution of the radiolabel was followed through the plasma membrane isolation procedure. The purity of isolated plasma membrane was assessed by enrichment of radiolabel, by electron microscopy, and by enzymatic assays for contaminating membranes. As judged from enrichment of radiolabel, a 37-fold purification of plasma membrane was obtained. We estimate that 80% of the total protein was from plasma membrane and 10% from membrane-associated actin.