59.7 kDa (530 aa) baculovirus core protein essential for oral infectivity in insect larvae (249). The Spodoptera exigua (Se)MNPV PIF-1 protein is present in ODVs (128), most likely anchored in the membrane by a conserved N-terminal transmembrane region (249). PIF-1 was not found in a proteomic analysis of AcMNPV ODVs (20), suggesting a low abundance. Together with PIF-2 it mediates binding of ODVs to epithelial midgut cells (203).
Ac120: ac120, unknown function
The putative gene product encoded by ac120 has a molecular mass of 9.5 kDa (82 aa). There is no information about the role of the ac120 product in the baculovirus life cycle. Homologs of ac120 can only be found in genomes of several other members of the genus Alphabaculovirus (Table 2).
Ac121: ac121, unknown function
A homolog of ac121 is only present in the genome of BmNPV (Table 2) and encodes a 6.7 kDa (58 aa) protein. There is evidence that Ac121 stimulates expression of the viral protein 39K (ac36) by up-regulation of IE1 (ac147) expression (78). Ac121 does not influence late gene expression (149).
Ac122: ac122, unknown function
Ac123: pk2, protein kinase 2
The gene pk2 is transcribed early as an 1.2 kb RNA and encodes the protein PK2 (24.9 kDa, 215 aa). PK2 contains six out of eleven motifs conserved among eukaryotic protein kinases (152). Truncation of the pk2 gene has no effect on the number, size, or appearance of viral plaques and on the kinetics of protein synthesis or protein phosphorylation profiles during virus infection in cultured Sf21 cells. PK2 mutants show no difference in infectivity or virulence in larval bioassays, neither in production of OBs as compared to wild type AcMNPV infection (152). PK2 prevents the phosphorylation of the eukaryotic translation initiation factor 2α (eIF2α). This phosphorylation is induced by stress, as caused by viral infection and inhibits protein synthesis in general (39). PK2 is a homolog of cellular eIF2α kinases, but is an inactive, truncated enzyme. By forming hetero-dimers with the cellular eIF2α kinases their phosphorylation activity is inhibited. Wild type AcMNPV shows a reduced eIF2α phosphorylation and increased translational activity, compared to a pk2 deletion mutant (51).
Ac124: ac124, unknown function
Ac125: lef-7, late expression factor 7
The lef-7 gene is transcribed early in infection from an initiation site 14 to 16 bp upstream of the putative translational start site and transcribed late in infection from a not determined initiation site more upstream (194). LEF-7 (26.6. kDa, 226 aa) is required for maximum late reporter gene expression (167). Deletion of lef-7 results in decreased BV and ODV production and DNA replication compared to the wild type virus infection (30). In BmNPV deletion of lef-7 also resulted in decreased levels of viral DNA replication (77). Furthermore, lef-7 is required for efficient homologous recombination in the presence of all other DNA replication genes (45).
Ac126: ac126, chitinase (ChiA)
The AcMNPV chitinase protein (ChiA) has a predicted molecular mass of 61.4 kDa (551 aa). ChiA accumulates in the endoplasmatic reticulum due to the presence of a signal peptide and a KDEL-retention signal (241, 261). Its release upon cell death is mediated by P10 (ac137) (261). ChiA is -together with the protease cathepsin (V-CATH; ac127) -required for disruption of the chitin skeleton of the host (96). The resulting liquefaction of the insect enables the efficient spread of viral occlusion bodies. ChiA is also prerequisite for processing of v-CATH from an inactive proenzyme (102). As a consequence deletion of chiA (+/-v-cath) from baculovirus expression vectors is used to reduce recombinant protein degradation (120). The chiA genes are present in many Alphabaculovi-ruses and some Betabaculoviruses and may have been picked up later in baculovirus evolution (Table 2 and Table 3).
Ac127: v-cath, cathepsin
The protease cathepsin (V-CATH) (predicted size 36.9 kDa, 323 aa) is activated from an inactive precursor by ChiA (ac126) and both proteins (V-CATH and ChiA) are required for the liquefa