Insects have evolved a system for storage of amino acids in which amino acids derived from dietary proteins during the last feeding larval stage are used to synthesize large amounts of a few well-defined proteins, called storage proteins. Storage proteins are hexamers, containing one or more types of subunits. In Manduca sexta, the subject of study in this application, there are three major storage proteins (SP); SP-1 or arylphorin, a protein rich in aromatic amino acids; SP-2, a protein rich in methionine; and SP-3, an as yet uncharacterized storage protein. During the last larval stadium, these proteins accumulate to very high levels in the blood and their mRNAs represent major species in the fat body, the site of synthesis. At the end of larval life SP-2 and SP-3 are taken back into the fat body and packaged as protein granules, while SP-1 remains in the blood. SP-1 is found equally in both sexes, whereas SP-2 and SP-3 occur in much higher concentrations in female blood and appear earlier in the blood of last instar female larvae than in male larvae. None of these proteins are found in adult hemolymph. THe storage proteins serve as important models for two fundamental questions in cell biology, viz, 1) how is the hormonal and developmental expression of their genes controlled; and 2) how are the storage proteins taken up by the fat body, and how does this system discriminate between the two classes of storage proteins? We propose to use the storage proteins to investigate these questions in a model system, the tobacco hornworm Manduca sexta, through pursuit of the following specific aims: 1) Determination of the protein (cDNA) and gene sequences for sp-2 and sp-3; 2) Investigation of potential cis-regulatory elements in the storage protein genes; 3) Isolation and characterization of transacting factors which recognize cis-regulatory elements in the storage protein genes; 4) Characterization of storage protein uptake up the fat body.
| Wang, X Y; Frohlich, D R; Wells, M A (1993) Polymorphic cDNAs encode for the methionine-rich storage protein from Manduca sexta. Insect Mol Biol 2:13-20 |
