Isolation of a calcium-binding phosphoprotein from the oocytes and hemolymph of the blood-sucking insect Rhodnius prolixus
1996
Silva-Neto, M.A.C. | Atella, G.C. | Fialho, E. | Paes, M.C. | Zingali, R.B. | Petretski, J.H. | Alves, E.W. | Masuda, H.
A novel calcium-binding phosphoprotein was isolated from the oocytes of the blood-sucking bug Rhodnius prolixus. This protein exhibits an apparent molecular mass of 18 kDa on gel filtration, but migrates as an 8-kDa band on N-[2-hydroxy- 1,1-bis(hydroxymethyl)ethyl]glycine/ SDS-polyacrylamide gels. It has a high content of serine (24% of the total number of residues), and phosphoserine is the sole amino acid phosphorylated in vivo. A similar protein was partially purified from the hemolymph. It resembles the oocyte form of the protein in its NH2-terminal sequence and its ability to be taken up by growing ovaries. 45Ca binding to the oocyte phosphoprotein was determined after SDS-polyacrylamide gel electrophoresis followed by blotting on nitrocellulose membranes. Titration of Ca2+-binding sites shows a high capacity (approximately equal to 50 mol/mol of protein), but a low affinity (K0.5 approximately equal to 10(-3) M). Based on these characteristics, we have named this protein Rhodnius calcium-binding phosphoprotein. It resembles phosvitin, a phosphoprotein present in the oocytes of nonmammalian vertebrates.
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