Molecular biology

Enveloped. Spherical to pleomorphic, about 80-100 nm in diameter. About 20% of of released viral particles already contains dsDNA genomes. This is due to an unique feature of spumaviruses in which the onset of reverse transcription of genomic RNA occurs before release rather than after entry of the new host cell like in other retroviruses.


Monopartite, linear, dimeric, ssRNA(+)/dsDNA (depending on the timing of reverse transcription) genome of about 12 kb, with a 5’-cap and a 3’poly-A tail. There are two long terminal repeats (LTRs) of about 600nt long at the 5’ and 3’ ends. The LTRs contain the U3, R, and U5 regions. There are also a primer binding site (PBS) at the 5’end and a polypurine tract (PPT) at the 3’end.

Spumaviruses are considered as promising vectors for the targeted delivery and expression of therapeutic genes or antigens.


The integrated provirus utilizes the promoter elements in the 5’LTR to drive transcription. This gives rise to the unspliced full length mRNA that will serve as genomic RNA to be packaged into virions or used as a template for translation of gag (Gag protein, p3).
The spliced mRNAs encode (pro)pol (PR, RT, RnaseH, IN) and env (Leader peptide, SU, TM). Incompletely spliced mRNAs contain cis-acting sequences that directly bind cellular factors to allow export from the nucleus into the cytoplasm.
The spumaviruses genome contains up to three additional ORFs encoding Bel1, Bel2 and Bel3 that are expressed predominantly by the internal promoter (IP) localized in the env gene. The cytoplasmic Bet protein consists of bel1 and bel2 spliced sequences.


  1. Virus attaches to host receptors through the SU glycoprotein. TM glycoprotein mediates fusion with cell membrane.
  2. Internalization and uncoating.
  3. ssRNA(+) or dsDNA genome enter the cell. ssRNA is copied into a linear dsDNA molecule by the reverse transcriptase.
  4. Nuclear entry of the viral dsDNA which is covalently and randomly integrated into the cell’s genome by the viral integrase (=provirus).
  5. Transcription of provirus to form viral RNAs and splicing.
  6. Translation of the RNAs to form precursor polyproteins.
  7. Late reverse transcription can occur.
  8. Assembly of the virion and packaging of the viral RNA genome.
  9. Budding predominantly into the endoplasmic reticulum (ER) and occasionally through the plasma membrane, most viral particles remain in the cell.
  10. Proteolytic processing of the precursors polyproteins by viral protease and maturation of the virions.

Matching UniProtKB/Swiss-Prot entries

27 entries grouped by strain (browse by keywords)

7 entries

Human spumaretrovirus (SFVcpz(hu)) (Human foamy virus) reference strain

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BEL1_FOAMVProtein Bel-1 (Transactivator of spumavirus) (Tas) (Transcriptional ...
BEL3_FOAMVProtein Bel-3
BET_FOAMVProtein Bet
ENV_FOAMVEnvelope glycoprotein gp130 (Env polyprotein)
GAG_FOAMVGag polyprotein (Pr71Gag)
POL_FOAMVPro-Pol polyprotein (Pr125Pol)
S1_FOAMVPutative uncharacterized protein S1

5 entries

Feline foamy virus (FFV) (Feline syncytial virus) reference strain

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BEL1_FFVProtein Bel-1 (Transactivator of spumavirus) (Tas) (Transcriptional ...
BET_FFVProtein Bet
ENV_FFVEnvelope glycoprotein gp130 (Env polyprotein)
GAG_FFVGag polyprotein
POL_FFVPro-Pol polyprotein (Pr125Pol)

5 entries

Simian foamy virus (isolate chimpanzee) (SFVcpz) reference strain

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BEL1_SFVCPProtein Bel-1 (Transactivator of spumavirus) (Tas) (Transcriptional ...
BET_SFVCPProtein Bet
ENV_SFVCPEnvelope glycoprotein gp130 (Env polyprotein)
GAG_SFVCPGag polyprotein (Pr71Gag)
POL_SFVCPPro-Pol polyprotein (Pr125Pol)

5 entries

Simian foamy virus type 1 (SFVmac) (SFV-1) reference strain

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BEL1_SFV1Protein Bel-1 (Transactivator of spumavirus) (Tas) (Transcriptional ...
BEL2_SFV1Protein Bel-2
ENV_SFV1Envelope glycoprotein gp130 (Env polyprotein)
GAG_SFV1Gag polyprotein (Pr71Gag)
POL_SFV1Pro-Pol polyprotein (Pr125Pol)

5 entries

Simian foamy virus type 3 (strain LK3) (SFVagm) (SFV-3)

Select_all Deselect_all  
BEL1_SFV3LProtein Bel-1 (Transactivator of spumavirus) (Tas) (Transcriptional ...
BEL2_SFV3LProtein Bel-2
ENV_SFV3LEnvelope glycoprotein gp130 (Env polyprotein)
GAG_SFV3LGag polyprotein (Pr71Gag)
POL_SFV3LPro-Pol polyprotein (Pr125Pol)