Foamy viruses (FV) are unusual among retroviruses since they require both Gag and Env structural proteins for particle egress. Recently significant progress has been made towards the mechanistic understanding of the viral release process, in particular that of retroviruses, and the viral domains and cellular pathways involved. However little is currently known about domains of FV structural proteins and cellular proteins engaged in this process. By mutational analysis of sequence motifs in prototype FV (PFV) Gag, bearing homology to known late assembly (L) domains, a PSAP motif with L domain function that was functionally interchangeable by heterologous L domains was identified. In contrast the inactivation of a PPPI motif had no significant influence on PFV particle release, although mutant viral particles displayed reduced infectivity. Similarly mutation of an evolutionary conserved YXXL motif revealed no classical L-domain function but resulted in release of noninfectious viruslike particles. Biochemical and electron microscopy analysis demonstrated that these mutant particles incorporated all viral structural proteins but contained aberrantly capsid structures, suggesting a role in capsid assembly for this PFV Gag sequence motif. In line with the mutational analysis, overexpression of dominant negative (DN) mutants and wild-type TSG101 but not the DN mutant of AIP-1/ALIX reduced PFV particle release and infectivity. Furthermore, DN mutants of Vps4A, Vps4B, and CHMP3 inhibited PFV egress and infectivity. Taken together these results demonstrate that PFV, like other viruses, requires components of the vacuolar protein sorting (VPS) machinery for egress and enters the VPS pathway through interaction with TSG101.Particle release from infected cells is one of the last steps in the retroviral replication cycle, which is accomplished by budding of viral particles across cellular membranes. On the viral side the orthoretroviral Gag polyprotein contains all the essential structural information for particle egress, and Gag expression by itself is sufficient for the release of viruslike particles (VLP). Although for some orthoretroviruses, such as Mason-Pfizer monkey virus (MPMV), coexpression of Env enhances viral particle release (37). In recent years significant advances have been made in the understanding of the mechanistic processes involved in the retroviral budding process. In particular the interaction domains of the viral Gag protein with essential cellular factors required during late stages of the budding process and involved in pinching off the viral particle from the cellular membrane have been identified. To date three consensus sequences of so-called viral late-budding or late-assembly (L) domains have been characterized. A P(T/ S)AP L-domain motif originally identified in the human immunodeficiency virus type 1 (HIV-1) Gag p6 domain (13), a PPXY L-domain motif first found in the Rous sarcoma virus (RSV) Gag p2b cleavage product (47) and a YPXL motif present in the equine infectious anemia virus (EIAV) G...

Background: During inflammation, β 2 -integrins mediate leukocyte adhesion to the endothelium accompanied by the activation of the spleen tyrosine kinase Syk.

No abstract

Unlike other retrovirus Gag proteins, the prototype foamy virus (PFV) p71gag protein is not processed into mature matrix (MA), capsid (CA), and nucleocapsid (NC) subunits. Little information about sequence motifs involved in FV capsid assembly and release is available. The recent analysis of candidate L-domain motifs in PFV Gag identified an evolutionarily conserved YXXL sequence motif with a potential function in capsid assembly. Here we provide support for the hypothesis that this motif does not function like a conventional L domain, by demonstrating that, unlike the PFV Gag PSAP L-domain motif, it cannot be functionally replaced by heterologous L-domain sequences. Furthermore, mutation of individual amino acids Y 464 , I 466 , L 467 , and L 469 , but not E 465 , to alanine led to reduced particle release and production of noninfectious, aberrant capsid structures, although relative structural protein incorporation and processing were not affected. In contrast, mutation of G 468 to alanine resulted in an intermediate, temperature-sensitive phenotype characterized by reduced particle release and reduced infectivity. Despite similar relative RNA genome incorporation for all mutants, analysis and quantification of particle-associated viral nucleic acids demonstrated defects in genomic reverse transcription for all the noninfectious mutants, a process that, unlike that of orthoretroviruses, in the case of FVs takes place in the virus-producing cell. In correlation with the reduced infectivity, the G 468 A mutant displayed an intermediate level of genomic reverse transcription. Taken together, these results demonstrate that the conserved YXXLGL motif in PFV Gag is involved in correct capsid assembly, which in turn is essential for reverse transcription of the FV genome.The orthoretrovirus Gag polyproteins are the driving force for viral particle assembly and release from infected host cells (reviewed in references 5 and 30). Typically, these polyproteins are processed in the newly formed viral particles during (or shortly after) budding into at least three mature subunits: MA, CA, and NC. Several functional domains within the polyprotein or the mature subunits have been identified. These include membrane-targeting (M) domains, located at or near the N terminus in the MA subunit, as well as interaction (I) domains, which are important for Gag oligomerization, and nucleic acid binding domains, which are required for viral genome packaging. The I domains and the nucleic acid binding domains are often located in the NC subunit. Furthermore, late assembly (L) domains have been characterized that are essential for late events in retrovirus budding, i.e., the pinching off and release of virions from infected host cells (reviewed in reference 3). At least three different late domain sequence motifs found in different virus families have been demonstrated to link the viral budding process to the cellular ubiquitylation machinery and the vacuolar protein sorting pathway. These are normally responsible for the sorting of carg...