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Mosquito transferrin, an acute-phase protein that is up-regulated upon infection
T Yoshiga1, V P Hernandez, A M Fallon
1The Department of Biochemistry and the Center for Insect Science, University of Arizona, Tucson, AZ 85721, USA.
Abstract:
When treated with heat-killed bacterial cells, mosquito cells in culture respond by up-regulating several proteins. Among these is a 66-kDa protein (p66) that is secreted from cells derived from both Aedes aegypti and Aedes albopictus. p66 was degraded by proteolysis and gave a virtually identical pattern of peptide products for each mosquito species. The sequence of one peptide (31 amino acids) was determined and found to have similarity to insect transferrins. By using conserved regions of insect transferrin sequences, degenerate oligonucleotide PCR primers were designed and used to isolate a cDNA clone encoding an A. aegypti transferrin. The encoded protein contained a signal sequence that, when cleaved, would yield a mature protein of 68 kDa. It contained the 31-amino acid peptide, and the 3' end exactly matched a cDNA encoding a polypeptide that is up-regulated when A. aegypti encapsulates filarial worms [Beerntsen, B. T., Severson, D. W. & Christensen, B. M. (1994) Exp. Parasitol. 79, 312-321]. This transferrin, like those of two other insect species, has conserved iron-binding residues in the N-terminal lobe but not in the C-terminal lobe, which also has large deletions in the polypeptide chain, compared with transferrins with functional C-terminal lobes. The hypothesis is developed that this transferrin plays a role similar to vertebrate lactoferrin in sequestering iron from invading organisms and that degradation of the structure of the C-terminal lobe might be a mechanism for evading pathogens that elaborate transferrin receptors to tap sequestered iron.
Insights
Mosquito cells secrete a protein similar to transferrin when exposed to bacteria. This mosquito transferrin may sequester iron, potentially hindering pathogen growth and survival.
Area of Science:
- Molecular Biology
- Immunology
- Parasitology
Background:
- Mosquito cells up-regulate protein production in response to heat-killed bacteria.
- A secreted 66-kDa protein (p66) is identified in both Aedes aegypti and Aedes albopictus cell cultures.
Purpose of the Study:
- To identify and characterize the secreted p66 protein.
- To investigate the function and evolutionary implications of mosquito transferrin.
Main Methods:
- Proteolytic degradation of p66 to analyze peptide products.
- Peptide sequencing and comparison to known protein families.
- Degenerate oligonucleotide PCR using conserved insect transferrin sequences to isolate A. aegypti transferrin cDNA.
- Bioinformatic analysis of the A. aegypti transferrin sequence.
Main Results:
- p66 exhibits conserved peptide fragments across mosquito species.
- A cDNA encoding A. aegypti transferrin was isolated, with the mature protein predicted at 68 kDa.
- The identified transferrin contains iron-binding residues in the N-terminal lobe but lacks them in the C-terminal lobe, which also shows deletions.
- The A. aegypti transferrin sequence is homologous to a previously identified upregulated polypeptide during filarial worm encapsulation.
Conclusions:
- The characterized protein is a mosquito transferrin with potential iron-sequestering capabilities, analogous to vertebrate lactoferrin.
- Structural modifications in the C-terminal lobe may represent an adaptation to evade pathogens that utilize transferrin receptors.