It is in reference to the => F18 (driver proteins) phosphoprotein is one of the most abundant core components and is essential for the assembly of mature virions. Basically, there is mass confusion surrounding F18 but there are many driver proteins as noted below.
Remember, all this was prior to these authors realizing that you could target flipped PS. The very last publication I will point out again, how Dr. Choe clearly says the trouble begins when PS flips to the outside.
Structure/Function Analysis of the Vaccinia Virus F18 Phosphoprotein, an Abundant Core Component Required for Virion Maturation and Infectivity?
Poxvirus virions, whose outer membrane surrounds two lateral bodies and a core, contain at least 70 different proteins. The F18 phosphoprotein is one of the most abundant core components and is essential for the assembly of mature virions. We report here the results of a structure/function analysis in which the role of conserved cysteine residues, clusters of charged amino acids and clusters of hydrophobic/aromatic amino acids have been assessed. Taking advantage of a recombinant virus in which F18 expression is IPTG (isopropyl-ß-d-thiogalactopyranoside) dependent, we developed a transient complementation assay to evaluate the ability of mutant alleles of F18 to support virion morphogenesis and/or to restore the production of infectious virus. We have also examined protein-protein interactions, comparing the ability of mutant and WT F18 proteins to interact with WT F18 and to interact with the viral A30 protein, another essential core component. We show that F18 associates with an A30-containing multiprotein complex in vivo in a manner that depends upon clusters of hydrophobic/aromatic residues in the N' terminus of the F18 protein but that it is not required for the assembly of this complex. Finally, we confirmed that two PSSP motifs within F18 are the sites of phosphorylation by cellular proline-directed kinases in vitro and in vivo. Mutation of both of these phosphorylation sites has no apparent impact on virion morphogenesis but leads to the assembly of virions with significantly reduced infectivity.
Vaccinia virus is the prototypic poxvirus and was the strain used in the successful vaccination campaign that led to the eradication of smallpox as a natural pathogen of humans. Poxvirus virions are large and structurally complex. Mature virions are approximately 350 by 250 nm in size and contain at least 70 different proteins, a lipid bilayer, and a double-stranded DNA genome of approximately 192 kb (4). Virion morphogenesis is quite unusual and occurs solely within the cytoplasm of infected cells. Initially, proteins destined for encapsidation within the virion accumulate in electron-dense zones known as factories or virosomes. At the periphery of these virosomes, membrane crescents appear. These crescents are composed of a single lipid bilayer that contains numerous integral membrane proteins (4); an external scaffold composed of the D13 protein supports the crescents as they enlarge and determines the size and curvature of the membrane (7, 29). As the membrane grows it becomes the delimiting boundary of the oval immature form of the virion (immature virion [IV]); the genome is encapsidated into the IV, most probably before the membrane seals. The D13 scaffold is then released from the IV, which mature into infectious, mature virions (MV). MV are brick-shaped and contain a dumbbell-shaped core; two lateral bodies of unknown function lie in the concavities of the core (4).
The outer boundary of the core is a proteinaceous shell which surrounds the genome, as well as the complete machinery for early transcription (4, 18). The proteins that comprise the core, as well as those found within the membrane, have been identified by a variety of biochemical, immunological, and mass spectrometric approaches (3, 21, 36). The function of many of these proteins has also been elucidated through the study of conditionally lethal viruses such as temperature-sensitive mutants or inducible recombinants in which the expression of individual gene products is IPTG (isopropyl-ß-d-thiogalactopyranoside) or TET dependent (6, 12, 14).
One of the very first core proteins shown to be essential for the production of infectious virions is an 11-kDa protein referred to in the literature as p11, F18, or F17 (8-10, 20, 37). This protein was originally annotated as the product of the F18R open reading frame (37), but in fact this reflected an annotation error; there are only 17 open reading frames in the HindIII F fragment. In this report, we will adhere to the original nomenclature and refer to the protein as F18 to provide continuity with most of the earlier literature. The abundant F18 protein is expressed at late times of infection; early reports indicated that ~27,000 copies of the protein are encapsidated per virion (25). The protein is encoded by all known poxviruses but has no recognizable homologs outside of the poxvirus family. The amino acid sequence of F18 gives no hint to its function; early biochemical studies suggested that F18 could multimerize and was associated with a weak affinity for DNA (10). The protein is highly basic (predicted pI of 9.37) and is known to undergo phosphorylation on two serine residues (9, 10).
In an initial study (37) regarding the general utility of inducer-dependent viruses, repression of the F18 protein was shown to compromise virion assembly. The proteolytic processing of core proteins was found to be incomplete, and immature particles with aberrant internal structures were seen. No mature virions assembled in the absence of F18. The phenotype of the IPTG-inducible recombinant makes it clear that F18 is dispensable for the formation of membrane crescents but essential for the proper filling of immature virions and their conversion to mature virions. Consistent with such a role, we have previously utilized immunoelectron microscopy to localize F18 throughout the lumen of immature virions and to a ring corresponding to the core boundary in mature virions (32). Because no temperature-sensitive mutants with lesions that affect F18 have been identified, we have gained no further insight into how F18 functions or into which residues/domains contribute to its role in virion assembly. We therefore undertook a thorough structure/function analysis of F18 in order to better understand its contributions to poxvirus morphogenesis and maturation.
Poxvirus Cell Entry: How Many Proteins Does it Take?
Bernard Moss
Regardless of the pathway or whether the MV or EV mediates infection, fusion is dependent on 11 to 12 non-glycosylated, transmembrane proteins ranging in size from 4- to 43-kDa that are associated in a complex. These proteins are conserved in poxviruses making it likely that a common entry mechanism exists. Biochemical studies support a two-step process in which lipid mixing of viral and cellular membranes is followed by pore expansion and core penetration. .. .. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3386626/
Lipid Membranes in Poxvirus Replication Jason P. Laliberte and Bernard Moss*
2.1.3. Entry of EVs and the EV-specific membrane
The EV contains an additional outer membrane compared to the MV. Relative to MVs, EVs were reported to be enriched in sphingomyelin and PS with lower amounts of phosphatidylinositol
So all above F18 driver proteins HAVE FLIPPED PS but that was before it was known that we should be targeting flipped PS. Now all are becoming aware. Dr. Choe knows, Dr. Rolf Brekken knows... Dr. Raymond Birge knows... Dr. Jedd Wolchok knows... Dr Gabrilovich knows.... many more in the know!
"Bavituximab is a first-in-class phosphatidylserine (PS)-targeting monoclonal antibody that is the cornerstone of a broad clinical pipeline." -- Big Pharmas nightmare... unless they are fortunate enough to have The Bavi Edge!
AI
