ISOLATION OF A CALCIUM-BINDING PHOSPHOPROTEIN FROM THE OOCYTES AND HEMOLYMPH OF THE BLOODSUCKING INSECT RHODNIUS-PROLIXUS

A novel calcium-binding phosphoprotein was isolated from the oocytes o f the blood-sucking bug Rhodnius prolixus. This protein exhibits an ap parent molecular mass of 18 kDa on gel filtration, but migrates as an 8-kDa band on 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 s imilar protein was partially purified from the hemolymph. It resembles the oocyte form of the protein in its NH2-terminal sequence and its a bility to be taken up by growing ovaries. Ca-45 binding to the oocyte phosphoprotein was determined after SDS-polyacrylamide gel electrophor esis followed by blotting on nitrocellulose membranes. Titration of Ca 2+-binding sites shows a high capacity (congruent to 50 mol/mol of pro tein), but a low affinity (K-0.5, congruent to 10(-3) M). Based on the se characteristics, we have named this protein Rhodnius calcium-bindin g phosphoprotein, It resembles phosvitin, a phosphoprotein present in the oocytes of nonmammalian vertebrates.