Mycoplasma phosphoenolpyruvate-dependent sugar phosphotransferase system: purification and characterization of the phosphocarrier protein

Structural evidence for the evolutionary divergence of mycoplasma from gram-positive bacteria: the histidine-containing phosphocarrier protein

BACKGROUND

The three-dimensional structures of histidine-containing phosphocarrier protein (HPr), a member of the phosphoenolpyruvate:sugar phosphotransferase system (PTS), have been determined from Gram-negative and Gram-positive bacteria. The structure of HPr reported here for Mycoplasma capricolum is the first protein structure to be determined for this class of organism. Comparative structural studies with the bacterial proteins highlight sequence-structure correlations relevant to proposals about the evolutionary origin of mycoplasmas.

RESULTS

The crystal structure of HPr from M. capricolum has been determined and refined at 1.8 A resolution, revealing the same overall fold as that of other HPrs of known structure. However, mycoplasma HPr resembles HPrs from Gram-positive bacteria more closely than those from Gram-negative bacteria. As in HPrs from Bacillus subtilis and Escherichia coli, the phosphoryl group carrier (His15) forms the N-terminal cap of a helix, but in contrast to the other crystal structures, the side chain of the adjacent Arg17 is conformationally disordered. A sulfate ion interacts with Ser46, a residue known to be phosphorylated in a regulatory manner.

CONCLUSIONS

The greater degree of structural similarity of the M. capricolum HPr to HPrs from Gram-positive rather than Gram-negative bacteria is consistent with the proposal that mycoplasma evolved from Gram-positive bacteria. The proposal that no major conformational transition is required for phosphorylation of the active-site histidine is reinforced by comparing the crystal structures with and without an anion in the active site. The conformational disorder of the Arg17 side chain suggests that its guanidinium group does not have to form specific interactions with other protein groups before phosphorylation at His15. The association of a sulfate ion with Ser46 serves as a model for HPr(Ser46-P). As there is no evidence of a conformational change accompanying Ser46 phosphorylation, the inhibitory effect of this event may be attributable to altered surface electrostatics.

Unique dicistronic operon (ptsI-crr) in Mycoplasma capricolum encoding enzyme I and the glucose-specific enzyme IIA of the phosphoenolpyruvate:sugar phosphotransferase system: cloning, sequencing, promoter analysis, and protein characterization

The region of the genome of Mycoplasma capricolum encompassing the genes for Enzymes I and IIAglc of the phosphoenolpyruvate:sugar phosphotransferase system (PTS) was cloned and sequenced. Examination of the sequence revealed a unique arrangement of the pts operon. In all other bacterial species characterized thus far, the gene encoding Enzyme I (ptsI) in the pts operon is located immediately downstream of the gene (ptsH) encoding HPr, a general energy coupling protein of the PTS. In M. capricolum, ptsH and ptsI reside on 2 distinct operons at separate loci on the chromosome (Zhu PP, Reizer J, Reizer A, Peterkofsky A, 1993, J Biol Chem 268:26531-26540). In the present work, it is shown that the Mycoplasma Enzyme I gene is preceded by an open reading frame homologous to the product of the Escherichia coli kdtB gene and is followed by the gene (crr) encoding Enzyme IIAglc. Northern blot analysis indicated that ptsI and crr constitute a dicistronic operon that includes an independent promoter for the crr gene. Primer extension studies established the transcription start sites for the ptsI and crr genes. The products of the ptsI and crr genes are homologous to previously sequenced Enzymes I and IIAglc proteins but are more similar to the counterpart proteins from gram-positive than to those from gram-negative organisms. The deduced amino acid sequence of the Mycoplasma Enzyme I shows that it differs from other Enzymes I by having fewer acidic amino acids and more basic, amidated, and aromatic amino acids. The deduced amino acid sequence of the Mycoplasma Enzyme IIAglc indicates that it is the shortest (154 residues) of the proteins in this class and it is the only Enzyme IIAglc with a tryptophan and a cysteine residue. In vitro sugar phosphorylation studies with extracts from E. coli and Bacillus subtilis and purified proteins indicated that the Mycoplasma HPr is not a phosphoacceptor from the E. coli Enzyme I, whereas the Mycoplasma Enzyme IIAglc accepts and transfers phosphate from both E. coli and B. subtilis PTS components.

Phosphoenolpyruvate:carbohydrate phosphotransferase systems of bacteria

Numerous gram-negative and gram-positive bacteria take up carbohydrates through the phosphoenolpyruvate (PEP):carbohydrate phosphotransferase system (PTS). This system transports and phosphorylates carbohydrates at the expense of PEP and is the subject of this review. The PTS consists of two general proteins, enzyme I and HPr, and a number of carbohydrate-specific enzymes, the enzymes II. PTS proteins are phosphoproteins in which the phospho group is attached to either a histidine residue or, in a number of cases, a cysteine residue. After phosphorylation of enzyme I by PEP, the phospho group is transferred to HPr. The enzymes II are required for the transport of the carbohydrates across the membrane and the transfer of the phospho group from phospho-HPr to the carbohydrates. Biochemical, structural, and molecular genetic studies have shown that the various enzymes II have the same basic structure. Each enzyme II consists of domains for specific functions, e.g., binding of the carbohydrate or phosphorylation. Each enzyme II complex can consist of one to four different polypeptides. The enzymes II can be placed into at least four classes on the basis of sequence similarity. The genetics of the PTS is complex, and the expression of PTS proteins is intricately regulated because of the central roles of these proteins in nutrient acquisition. In addition to classical induction-repression mechanisms involving repressor and activator proteins, other types of regulation, such as antitermination, have been observed in some PTSs. Apart from their role in carbohydrate transport, PTS proteins are involved in chemotaxis toward PTS carbohydrates. Furthermore, the IIAGlc protein, part of the glucose-specific PTS, is a central regulatory protein which in its nonphosphorylated form can bind to and inhibit several non-PTS uptake systems and thus prevent entry of inducers. In its phosphorylated form, P-IIAGlc is involved in the activation of adenylate cyclase and thus in the regulation of gene expression. By sensing the presence of PTS carbohydrates in the medium and adjusting the phosphorylation state of IIAGlc, cells can adapt quickly to changing conditions in the environment. In gram-positive bacteria, it has been demonstrated that HPr can be phosphorylated by ATP on a serine residue and this modification may perform a regulatory function.

The phosphoenolpyruvate:sugar phosphotransferase system in gram-positive bacteria: properties, mechanism, and regulation

This review consists of three major sections. The first and largest section reviews the protein constituents and known properties of the phosphotransferase systems present in well-studied Gram-positive bacteria. These bacteria include species of the following genera: (1) Staphylococcus, (2) Streptococcus, (3) Bacillus, (4) Lactobacillus, (5) Clostridium, (6) Arthrobacter, and (7) Brochothrix. The properties of the different systems are compared. The second major section deals with the regulation of carbohydrate uptake. There are four parts: (1) inhibition by intracellular sugar phosphates in Staphylococcus aureus, (2) PTS-mediated regulation of glycerol uptake in Bacillus subtilis, (3) competition for phospho-HPr in Streptococcus mutans, and (4) the possible involvement of protein kinases in the regulation of sugar uptake via the phosphotransferase system. The third section deals with the phenomenon of inducer expulsion. The first part is concerned with the physiological characterization of the phenomenon; then the consequences of unregulated uptake and expulsion, a futile cycle of energy expenditure, are considered. Finally, the biochemistry of the protein kinase and the protein phosphate phosphatase system, which appears to regulate sugar transport via the phosphotransferase system, is defined. The review, therefore, concentrates on the phosphotransferase system, its functions in carbohydrate transport and phosphorylation, the mechanisms of its regulation, and the mechanism by which it participates in the regulation of other physiological processes in the bacterial cell.

The mycoplasmas

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Mycoplasma phosphoenolpyruvate-dependent sugar phosphotransferase system: glucose-negative mutant and regulation of intracellular cyclic AMP

Glucose-negative mutants of Mycoplasma capricolum were selected for growth on fructose in the presence of the toxic glucose analog alpha-methyl-D-glucopyranoside. The mutants are defective in the phosphoenolpyruvate:sugar phosphotransferase system for glucose. One mutant, pts-4, was studied in detail. It lacks the glucose-specific, membrane-bound enzyme II, IIGlc, as well as the general, low-molecular-weight, phosphocarrier protein, HPr. In place of the latter, however, it has a fructose-specific protein, HPrFru. Consistent with these changes, the mutant lost the ability to grow on glucosamine and maltose but retained its ability to grow on sucrose. In the glucose-negative mutant, glucose did not regulate the intracellular concentration of cyclic AMP. The intracellular concentration of cyclic AMP in M. capricolum is regulated by the presence of metabolizable sugars. In the wild-type, both glucose and fructose reduced the intracellular concentration of cyclic AMP; however, in the glucose-negative mutant, glucose no longer regulated the intracellular level of cyclic AMP.

Mycoplasma phosphoenolpyruvate-dependent sugar phosphotransferase system: purification and characterization of enzyme I

The Mycoplasma phosphoenolpyruvate-dependent sugar phosphotransferase system consists of three components: a membrane-bound enzyme II, a soluble phosphocarrier protein (HPr), and a soluble enzyme I. The soluble enzyme I was purified by ammonium sulfate fractionation; Bio-Gel P-10 gel filtration; acid precipitation; diethylaminoethyl-Bio-Gel A; and Bio-Gel HTP column chromatography. The enzyme I was shown to be homogeneous by electrophoresis in a pH 8.9 non-sodium dodecyl sulfate gel and by isoelectric focusing. Whereas the protein moved as a single component in both the non-sodium dodecyl sulfate gel and isoelectric focusing, on sodium dodecyl sulfate gels, it moved as three subcomponents. The molecular weights of the three subunits, alpha, beta, and gamma, were 44,500, 62,000 and 64,500, respectively. The holoprotein moved as a single component, in the region of 220,000 daltons, in a Bio-Gel A 0.5-agarose column. The molar ratio of subunits was estimated to be 2alpha:1beta:1gamma. The elution characteristics on a diethylaminoethyl column at pH 7.4 and 6.8, acid precipitation data, and amino acid composition indicated that the protein is acidic. Isoelectric focusing occurred at pH 4.8. N-terminal amino acids determined by the dansyl chloride method indicated that glycine, alanine, and tyrosine are N-terminal amino acids of the three subunits. Although the protein was stable for at least 14 months at -20 degrees C, it was irreversibly inactivated by the thiol reagent N-ethyl-maleimide.