rotein. The gene is not essential for virus infection and replication at least in vitro as viable ac18 mutants are formed in bioreactors (139). The lethal dose was not affected by deleting ac18, but time to death was increased (275). Which role ac18 plays, is still unclear.
Ac19: ac19, unknown function
This gene encodes a hypothetical protein with a calculated mass of 12.2 kDa (108 aa). Although homologs are found in other baculoviruses (Table 2), the function of the gene product is unknown. This gene is represented in the transcriptome (287).
Ac20/21: actin rearrangement inducing factor-1
The gene ac20 was identified in 1994 (9) and had the potential to encode a partial homolog of ac21. Resequencing of these ORFs showed that ac20 and ac21 in fact form one ORF (92). The ac20/ac21 fusion gene codes for the 47.7 kDa (417 aa) actin rearrangement inducing factor 1 (ARIF-1). The gene is expressed after transactivation by IE-1, weakly from 2 h p.i., more abundantly after 4-6 h p.i., and not detectably at 12 h p.i. (237). ARIF-1 is a tyrosine phosphorylated protein and induces rearrangement of the actin skeleton (237) by interacting with filamentous actin (F-actin) at the plasma membrane (53). Deletion of ARIF-1 interfered with F-actin accumulation at the plasma membrane, but not with the formation of early actin cables and nuclear F-actin accumulation (53, 237). Homologs are only present in Alphabaculoviruses (Table 2).
Ac22: pif-2, per os infectivity factor 2
Ac22 encodes the 43.8 kDa (382 aa) PIF-2 protein conserved in all baculoviruses and essential for oral infectivity of midgut cells (223). Hence, PIF-2 is a per os infectivity factor. PIFs are not needed when the virus is injected into the hemolymph (as for all four known baculovirus PIF proteins). Proteomic analysis showed the presence of PIF-2 in ODVs (20) and it has a predicted N-terminal membrane anchor (249). PIF-2 is thought to be involved in binding of ODVs to midgut epithelial cells, and possibly associates with PIF-1 (203). PIF-2 is a highly conserved protein belonging to the baculovirus core genes and is often used in baculovirus phylogeny.
Ac23: ac23, copia-like envelope protein
The gene ac23 is a truncated, non-functional homolog of the baculovirus F-protein present in group Ⅱ NPVs of the Alphabaculoviruses and in Beta-, and Deltabaculoviruses. The F-protein homolog or F-like protein Ac23 (79.9 kDa, 690 aa) does not function as fusion protein, as it lacks a functional furin cleavage site, but it may have other functions (219). An AcMNPV-mutant lacking ac23 showed that the gene is not essential for either infection, virus propagation or BV production, but the mutant killed T. ni larvae slower than wild type virus, suggesting that the F-like Ac23 protein is a viral pathogenicity factor in vivo (173).
Ac24: pkip, protein kinase interacting protein
The gene ac24 or pkip is a late gene and encodes a protein kinase-interacting protein (PKIP; 19.2 kDa, 169 aa) (62). PKIP interacts with PK-1 (see ac10) in virus-infected cells and stimulates activity of PK-1 (62). A temperature sensitive pkip mutant showed neither BV production nor VL gene expression, but intracellular nucleocapsids of this mutant structurally resembled those of the wild type AcMNPV (182).
Ac25: dbp, ssDNA-binding protein
Homologs of the dbp gene have been identified in all sequenced baculovirus genomes, except the dipteran CuniNPV (205). DBP (36.6 kDa, 316 aa) is expressed as an early gene product (204), which is essential for the production of viable virions. However, it is not required for synthesis of viral DNA nor for expression of viral genes (231, 267). DBP has a tight association with subnuclear structures and has high affinity for ssDNA. It has both DNA unwinding and renaturation activities and may be involved in the processing of replication intermediates (188).
Ac26: ac26, unknown function
The ac26 gene encodes a protein with a theoretical molecular mass of 14.6 kDa (129 aa) and has a conserved domain with unknown function in the NCBI Conserved Domain Database (CDD) (176). The gene appears t