Structural characterization of the lipovitellin from the shrimp Macrobrachium borellii

In oviparous species, proteins and lipids are found in the vitellus forming lipoproteins called lipovitellins. They are an important energy source for embryos development and larvae growth and survival. We have previously isolated and partially characterized the sole egg cytosolic lipovitellin from the freshwater shrimp Macrobrachium borellii. It is a native protein of 440 kDa, composed of two subunits of 94 and 112 kDa. In the present work we studied size, shape and structure of M. borellii lipovitellin using electron microscopy, crosslinking reagents, MALDI-TOF, circular dichroism, fluorescence and partial proteolysis. The results showed that lipovitellin has a quasi spherical morphology with an estimated diameter of 18.5+/-3.5 nm. It appears to be composed of two subunits of 94 kDa, and one of 112 kDa. The larger subunit is more susceptible to trypsinolysis, indicating that it is less compactly folded and/or more exposed to the aqueous medium than the 94 kDa subunits. The hetero-trimer is held together by non-covalent interactions. Peptide mass fingerprinting by MALDI-TOF, produced 42 polypeptides matching to a vitellogenin of a related species (Macrobrachium rosenbergii). Circular dichroism indicated that this protein contains 35.7% alpha-helix, 16.6% beta-sheet and 20% turns. Tryptophan fluorescence emission, at a maximum of 334 nm, indicated that the environment polarity of these aromatic residues is similar to that of other crustacean lipoproteins.