Polyhedrin structure.

Polyhedrin has evolved two highly specialized functions. Firstly, it forms a protective crystal around the virus and secondly it resists solubilization except under strongly alkaline conditions similar to those found in the insect midgut. Both of these properties allow the virus to remain viable for many years outside the insect host. Although polyhedrin and granulin can vary by about 50% in amino acid sequence, many of their structural features are highly conserved, reflecting the similar function and biochemical properties of these proteins. By comparing the sequences, domains within the gene which evolve rapidly have been identified and molecular phylogenies have been proposed. Information on predicted secondary structure has also been obtained and some insight into the possible role of codon usage in baculovirus function has also been gained from the sequence information. In addition to the conserved structural properties of the polyhedrin protein, there is a conserved regulatory process which results in the synthesis of massive amounts of polyhedrin. This process is probably governed by a virus-specific RNA polymerase. A potential regulatory signal shared by all these genes has been identified upstream from the polyhedrin gene. A number of additional granulin and lepidopteran polyhedrin sequences will certainly be forthcoming because of the ease with which these genes are identified by cross-hybridization with available related probes. However, of special interest will be sequences from dipteran and hymenopteran polyhedrins which will add greatly to our understanding of the constraints governing polyhedrin structure and diversity. Another logical step in their study will be to examine polyhedrin quaternary structure utilizing X-ray crystallography. Additional areas of future emphasis will probably focus on the regulation of polyhedrin synthesis. Elucidation of the regulatory signals governing transcription of these genes are of prime interest as are complementary studies on the characterization of the RNA polymerase which transcribes these genes.