Changes in viral protein function that accompany retroviral endogenization

doi:10.1073/pnas.0704313104

Changes in viral protein function that accompany retroviral endogenization

Laboratory of Cellular and Molecular Regulation, National Institute of Mental Health, Bethesda, MD 20892

Edited by John M. Coffin, Tufts University School of Medicine, Boston, MA, and approved August 20, 2007 (received for review May 8, 2007)

Abstract

Endogenous retroviruses (ERVs) are the remnants of ancient retroviral infections of germ cells and have been maintained in whole or part as heritable genomic elements. The last known endogenization events occurred several million years ago, and therefore stepwise analysis of retroviral endogenization has not been possible. A unique opportunity to study this process became available when a full-length ERV isolated from koalas (KoRV) was shown to have integrated into their germ line within the past 100 years. Even though KoRV shares 78% nucleotide identity with the exogenous and highly infectious gibbon ape leukemia virus (GALV), the infectivity of KoRV, like that of other ERVs, is substantially lower than that of GALV. Differences in the protein coding regions of KoRV that distinguish it from GALV were introduced into the GALV genome, and their functional consequences were assessed. We identified a KoRV gagpol L domain mutation as well as five residues present in the KoRV envelope (env) that, when substituted for the corresponding residues of GALV, resulted in vectors exhibiting substantially reduced titers similar to those observed with KoRV vectors. In addition, KoRV env protein lacks an intact CETTG motif that we have identified as invariant among highly infectious gammaretroviruses. Disruption of this motif in GALV results in vectors with reduced syncytia forming capabilities. Functional assessment of specific sequences that contribute to KoRV's attenuation from a highly infectious GALV-like progenitor virus has allowed the identification of specific modifications in the KoRV genome that correlate with its endogenization.

Footnotes

^‡To whom correspondence should be addressed at:
Laboratory of Cellular and Molecular Regulation, National Institute of Mental Health, 49 Convent Drive, Bethesda, MD 20892.
E-mail: eidenm{at}mail.nih.gov
Author contributions: N.M.O. and M.V.E. designed research; N.M.O., H.S., and M.V.E. performed research; I.A.K. contributed new reagents/analytic tools; N.M.O., H.S., I.A.K., and M.V.E. analyzed data; and N.M.O., I.A.K., and M.V.E. wrote the paper.
↵*Present address: Institute of Cell and Molecular Science, Queen Mary University of London, White Chapel, London E1 2AT, United Kingdom.
↵ ^†Present address: Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
↵ § GALV SEATO was the source of GALV vector components used in these experiments.
Abbreviations:

KoRV,

Koala retrovirus;

GALV,

gibbon ape leukemia virus;

MLV,

murine leukemia virus;

FeLV,

feline leukemia virus;

WMV,

woolly monkey virus;

ERV,

endogenous retrovirus;

gagpol,

group-specific antigen and polymerase;

env,

envelope;

RBD,

receptor binding domain;

PiT1,

inorganic phosphate transporter 1.