Toward the elucidation of the mechanism of attachment and entry of malaria sporozoites into cells: synthetic polypeptides from the circumsporozoite protein of Plasmodium falciparum bind Ca2+ and interact with model phospholipid membranes

Chelation of Ca²⁺ ions by a peptide from the repeat region of the Plasmodium falciparum circumsporozoite protein

BACKGROUND

Elegant efforts towards the determination of the structural tendencies of peptides derived from the Plasmodium falciparum circumsporozoite protein allowed the proposal of a left-handed helical conformation for this protein. The use of circular dichroism and Fourier-transformed infrared spectroscopy applied to various peptides derived from this protein, indicated that they bind Ca²⁺ ions in helical environments. The essential role of calcium in cell function and biological mechanisms is well known. It influences the development of several stages of the P. falciparum parasite. However, there is very little knowledge regarding calcium coordination to circumsporozoite proteins. In the present investigation the chelation of Ca²⁺ by the (NANPNVDP)₃NANP peptide, which contains the first seven 4-amino-acid blocks of the repeat region of the P. falciparum circumsporozoite protein, is tested with the use of circular dichroism and nuclear magnetic resonance spectroscopies. Spectroscopy-based solution conformations of the Ca-bound peptide are also determined.

METHODS

NMR spectroscopy and circular dichroism were used to test Ca²⁺ coordination by the peptide (NANPNVDP)3NANP. Solution conformations for the Ca-bound peptide were determined through molecular dynamics calculations.

RESULTS

The NMR spectra collected for (NANPNVDP)₃NANP indicate that the signals generated by some of the amino acids located at its C-terminal end are shifted from their original positions upon Ca²⁺ addition. The solution conformations determined for the Ca-bound peptide indicate that the metal ion can be either six- or seven-coordinate.

CONCLUSIONS

The investigation described herein strongly supports the coordination of Ca²⁺ ions to some of the amino acids located at the C-terminus of the peptide (NANPNVDP)₃NANP. The solution conformations determined for the Ca-bound congener of this peptide display many structural features associated to Ca-binding proteins.

Malaria vaccine development using synthetic peptides as a technical platform

The review covers the development of synthetic peptides as vaccine candidates for Plasmodium falciparum- and Plasmodium vivax-induced malaria from its beginning up to date and the concomitant progress of solid phase peptide synthesis (SPPS) that enables the production of long peptides in a routine fashion. The review also stresses the development of other complementary tools and actions in order to achieve the long sought goal of an efficacious malaria vaccine.

Release of malaria circumsporozoite protein into the host cell cytoplasm and interaction with ribosomes

To date, the circumsporozoite (CS) protein has been implicated in guiding malaria sporozoites to the liver [Cerami et al., Cell 70, 1992, 1021-1033]. Here we show that shortly after invasion, P. berghei and P. yoelii sporozoites lie free in the invaded cell and release considerable amounts of CS protein into the cytoplasm. The intracytoplasmic deposition of CS protein begins during the attachment of the sporozoite to the host cell surface and reaches its peak during the first 4-6 h after invasion. Initially, the CS protein spreads over the entire cytoplasm of the infected cell where it interacts with cytosolic as well as endoplasmic reticulum-associated ribosomes. During the subsequent development of the parasites to exoerythrocytic forms, the CS protein binding becomes gradually restricted to ribosomes lining the outer membrane of the nuclear envelope of the host cell. The distribution pattern of the parasite-released CS protein in the host cell cytoplasm is independent of the permissiveness of the host cell for the development of the parasites to exoerythrocytic forms. It requires neither the host cell metabolism nor does it involve the endocytotic machinery. Recombinant P. falciparum CS protein interacts with RNAse-sensitive sites on endoplasmic reticulum-associated ribosomes as shown by microinjection and immunoelectron microscopy. The generalized interaction of the CS protein with host cell ribosomes suggests that the CS protein has an intracellular function during the hepatic phase in the life cycle of Plasmodium and may also explain the generation of a CD8+ T cell response in the course of rodent malaria infections.

Interaction of phospholipids with proteins and peptides. New advances III

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins and peptides. 2. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecules is also discussed.

Characterization of murine monoclonal antibodies against a repetitive synthetic peptide from the circumsporozoite protein of the human malaria parasite, Plasmodium falciparum

Monoclonal antibodies (mAbs) were raised in mice against the synthetic peptide (NANP)40, consisting of 40 (NANP) repeats of the circumsporozoite (CS) protein of the human malaria parasite, Plasmodium falciparum, and characterized. (i) Five of these mAbs recognized the P. falciparum CS protein in western blot experiments and in immunofluorescence assays using different preparations of sporozoites. The remaining two mAbs (CT3.2 and CT3.3, both IgG1) gave negative results by both techniques. (ii) When the anti-(NANP)40 peptide mAbs were functionally tested in vitro to assess their ability to inhibit the attachment and penetration of the parasites into cultured human liver cells, six of them exhibited inhibitory activities ranging between 66 and 90%. CT3.2 mAbs, also, inhibited sporozoite attachment and penetration, despite the negative results by immunofluorescence and western blot experiments. However, when immunofluorescence was repeated in the presence of calcium, CT3.2 did reveal a positive recognition of P. falciparum sporozoites, suggesting that this mAb could recognize the (NANP) sequence when calcium was bound to the repetitive peptide. (iii) Furthermore, the binding of an anti-(NANP)40 IgM mAb (CT1) to the solid-phase peptide was not inhibited by preincubation of the peptide with a mAb against the P. falciparum CS protein. (iv) Finally, one anti-(NANP)40 IgG1 mAb (CT3.1) was unable to bind to the shorter (NANP)3 peptide, although it recognized the (NANP)40 peptide and the P. falciparum CS protein. The results presented here suggest that heterogeneous antibody populations are produced upon immunization of mice with (NANP)40 synthetic peptide and that epitopes different from those simply related to the linear (NANP) amino acid sequence are likely to be present in long (NANP)n constructs as well as in the repetitive domain of the P. falciparum CS protein.