Manduca sexta fat body during the larval-pupal transformation: A structural and biochemical study.

The insect fat body is an organ that plays major roles in metabolism, storage and transport of nutrients. Of particular importance to the insect are changes that occur during the larval-pupal transformation as the insect switches from ingesting and storing nutrients to utilizing stored nutrients. Because much is known about the biochemistry of Manduca sexta during this time but little was known of the structure of the fat body, the structure of the fat body of M. sexta during the final, i.e. the fifth, larval stadium was studied using light and electron microscopy. Changes in structure of the fat body agreed with known biochemical changes that occur during the final larval stadium. Some changes included: accumulation of lipid and carbohydrate stores; acquisition of protein granules and urate; and development and loss of rough endoplasmic reticulum, Golgi complex and the plasma membrane reticular system. The protein granules appear to serve as a store of amino acids for adult proteins. In order to better understand the role of the high molecular weight larval serum proteins (LSPs) found in the hemolymph and fat body, native polyacrylamide gel electrophoresis of fat body homogenates and hemolymph was used to indicate the relative prevalence of each of the high molecular weight larval serum proteins found in M. sexta during the final larval stadium. LSPs 2 and 3, found in greater amounts in females than males, appeared in the hemolymph later than LSP4 and accumulated in the fat body to a greater extent than did LSP4. LSPs were isolated from both hemolymph and fat body. Antisera were raised against LSP3 and LSP4 (arylphorin). Screening of a cDNA expression library with antiserum to LSP4 yielded cDNA clones for the two subunits of LSP4; these were sequenced to obtain the LSP4 subunit primary structures. The two sequences are very similar, with tyrosine and phenylalanine residues being distributed fairly evenly throughout the proteins. When compared with known protein sequences, LSP4 was found to be homologous to arthropod hemocyanin proteins.