Why is there sequence similarity between insect yolk proteins and vertebrate lipases?

The major proteins stored in the yolk of developing oocytes are thought to provide a nutritional store for utilization during embryogenesis. They seem to fall into two major families of proteins. The first are called vitellogenins and are found in frog, chicken, nematode, fish, and some insects such as the boll weevil. The other group are called yolk proteins and are found in dipteran insects such as fruitfly, housefly, fleshfly, and blue-bottles. Both groups are the major proteins found in the oocyte and are female-specific proteins endocytosed from the serum or hemolymph. The yolk protein group were found to have sequence similarity to the triacylglycerol lipases and lipoprotein lipases of vertebrates, including rat, pig, and human. The yolk proteins do not have lipase activity, but the sequences conserved between yolk proteins and lipases surround the active site where there are interactions with lipids. The likely reason for the presence of this domain in the yolk proteins is to bind a steroid hormone in a storage form conjugated to lipids. This permits the storage of the hormone in an inactive form until the yolk proteins are degraded, when it can be released from its conjugate to induce developmental decisions in embryogenesis. They may also transport lipids into the oocyte for use in embryogenesis. Whilst the vitellogenin family of proteins do not share this homology with the lipases they do have similarity to the human serum protein, apolipoprotein B, which also has a role in binding lipids. These findings are discussed in relation to the evolution and functions of lipases, apolipoproteins, vitellogenins, and yolk proteins. Experiments aimed at isolating genes encoding lipases in insects and at further elucidating the function of the yolk proteins are suggested.