Human endogenous retrovirus HERV-K(HML-2) encodes a stable signal peptide with biological properties distinct from Rec.
BACKGROUND
The human endogenous retrovirus HERV-K(HML-2) family is associated with testicular germ cell tumors (GCT). Various HML-2 proviruses encode viral proteins such as Env and Rec.


RESULTS
We describe here that HML-2 Env gives rise to a 13 kDa signal peptide (SP) that harbors a different C-terminus compared to Rec. Subsequent to guiding Env to the endoplasmatic reticulum (ER), HML-2 SP is released into the cytosol. Biochemical analysis and confocal microscopy demonstrated that similar to Rec, SP efficiently translocates to the granular component of nucleoli. Unlike Rec, SP does not shuttle between nucleus and cytoplasm. SP is less stable than Rec as it is subjected to proteasomal degradation. Moreover, SP lacks export activity towards HML-2 genomic RNA, the main function of Rec in the original viral context, and SP does not interfere with Rec's RNA export activity.


CONCLUSIONS
SP is a previously unrecognized HML-2 protein that, besides targeting and translocation of Env into the ER lumen, may exert biological functions distinct from Rec. HML-2 SP represents another functional similarity with the closely related Mouse Mammary Tumor Virus that encodes an Env-derived SP named p14. Our findings furthermore support the emerging concept of bioactive SPs as a conserved retroviral strategy to modulate their host cell environment, evidenced here by a "retroviral fossil". While the specific role of HML-2 SP remains to be elucidated in the context of human biology, we speculate that it may be involved in immune evasion of GCT cells or tumorigenesis.
Ruggieri
Alessia
A
Department of Human Genetics, Medical Faculty, University of Saarland, Homburg, Germany. alessia_ruggieri@med.uni-heidelberg.de
Maldener
Esther
E
Sauter
Marlies
M
Mueller-Lantzsch
Nikolaus
N
Meese
Eckart
E
Fackler
Oliver T
OT
Mayer
Jens
J
D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000818:Animals; D002466:Cell Nucleolus / Q000235:genetics / Q000378:metabolism; D020077:Endogenous Retroviruses / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000235:genetics / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D051379:Mice; D008969:Molecular Sequence Data; D041681:NIH 3T3 Cells; D021382:Protein Sorting Signals; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D016601:RNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D016415:Sequence Alignment; D014759:Viral Envelope Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
C485839:ERVK-6 protein, human endogenous retrovirus; D021382:Protein Sorting Signals; D016601:RNA-Binding Proteins; D014759:Viral Envelope Proteins