Physical and chemical properties of microvitellogenin. A protein from the egg of the tobacco hornworm moth, Manduca sexta.

Microvitellogenin belongs to a new class of low molecular weight female-specific proteins in insects. The protein is found in the hemolymph (blood) and egg of the tobacco hornworm, Manduca sexta. The isolation of microvitellogenin has been achieved by a combination of gel permeation, cation-exchange, and adsorption chromatographic steps. Microvitellogenin is synthesized by the fat body and appears in the hemolymph 17 days before adult emergence, or 16 days before the onset of egg development. The protein is sequestered from the hemolymph into the egg where it accumulates to a relatively high concentration. The proteins isolated from the hemolymph and the egg are identical in their molecular weight, amino acid compositions, isoelectric points, circular dichroic spectra, immunological properties, and NH2-terminal amino acid sequence. Thus, microvitellogenin does not seem to undergo any modifications before or after it is sequestered in the egg. In solution, the protein exists in a monomeric form and has a secondary structure composed of approximately 38% alpha-helix, as estimated by CD analysis. The CD spectrum of microvitellogenin is unusual in that it has a strong positive band between 220 and 240 nm that may be due to contributions from the aromatic amino acid residues. Unlike the major egg yolk protein of insects, vitellogenin, microvitellogenin does not contain measurable carbohydrate or lipid, and has no immunological, chemical, or physical similarities to vitellogenin. The amino acid composition of microvitellogenin is low in cysteine, but is rich in aspartate. The sex specificity of the protein and its accumulation in the egg justifies the name microvitellogenin, first given to an analogous protein in the egg of the giant silkmoth, Hyalophora cecropia.