Pseudotyped retroviral vectors combine the advantages of broad host range,
high expression, stable chromosomal integration, and ease of preparation. T
hese vectors greatly facilitate delivery into mammalian cells of sequences
encoding individual peptide inhibitors-including those with therapeutic uti
lity-and inhibitor libraries. However, retroviral vectors vary in behavior,
particularly with respect to expression levels in different cell lines. Ex
pression level is especially important in transdominant experiments because
the concentration of an inhibitor (for example, an expressed peptide) is o
ne of the key determinants in the degree of complex formation between the i
nhibitor and its target. Thus, inhibitor concentration should have an impac
t on the expressivity and/or penetrance of an induced phenotype. Here, we c
ompare several retroviral vectors and human cell lines for relative express
ion levels using a green fluorescent protein reporter. We show for a subset
of these lines that cellular protein concentrations produced by single-cop
y vectors range up to about 2 muM. We also examine other variables that con
tribute to expression level, such as the nature of the expressed protein's
carboxy terminus. Finally, we test the effect of increased concentration on
phenotype with a nine-amino-acid peptide derived from the human papilloma
virus protein E7 which overcomes E7-mediated cell growth.