ERV3 in placenta

[Ruhlen Rachel]

I'm attempting to read and interpret an article from _Virology_, vol 211.
The title is "Abundance of an Endogenous Retroviral Envelope Protein in
Placental Trophoblasts Suggests a Biological Function".  The authors were
terse and the article is confusing in many places.  
If anyone is familiar with the article, and understands it, I would like to
know why was a fusion protein used to immunize the rabbits (for antibodies)
instead of just the TM protein?  What were the rabbits immunized with, 
precisely--one with the peptide sequence, and one with the GST, or were
they also immunized against the fusion protein?  

If you could answer my questions, or need further clarification of what I'm
asking, please let me know.

Rachel