Activities of HERV-K components: Capacity of the human genome to yield an infectious retrovirus

Abstract

Human Endogenous Retrovirus K (HERV-K) is the most recently acquired and intact retrovirus in the human genome. Throughout the course of human evolution, the HERV-K genome has incurred mutations, which in the absence of selective pressure for the host to maintain intact viral genes have accumulated. Thus, the majority of the HERV-K elements that exist in the human genome today contain mutations that render them nonfunctional.;No single HERV-K provirus in the human genome today appears to be infectious. Thus we decided to study function of individual HERV-K components. We investigated the abilities of Gag protein from three HERV-K proviruses to support production of virus-like particles and viral infectivity. The Gags of HERV-K101 and HERV-K108 allowed particle production and infectivity. In contrast, HERV-K113 Gag did not support particle production or virus infectivity due to a single amino acid substitution (I516M), in the extreme C-terminus of the CA protein within Gag. Our results that this domain is a major determinant in HERV-K particle assembly, adds to mounting evidence that this region of CA is crucial to retrovirus assembly in general.;We also investigated the expression and localization of the HERV-K Env glycoprotein. HERV-K Env is unusual in that the first 87 amino acids at its amino terminus are identical to those of the RNA nuclear export factor Rec. It was unclear where the signal peptide was located for this surface protein. In addition, there are 9 in frame ATG codons upstream of the first N-linked glycosylation site, none of which is positioned in an optimal Kozak context to initiate translation. In this thesis, we address where the HERV-K Env signal peptide is positioned in the protein, where translation initiation occurs, and how two proteins with distinct localization and function share a large stretch of primary sequence. We found that the first methionine codon initiates Env translation despite our finding that the signal peptide does not begin until the sixth methionine codon positioned at amino acid 76. Furthermore, we found that splicing at the Rec 5'-ss located within the SP prevents ER localization of the nascent protein.