FIG. 4.

Schematic representation of the Fv1 and Ref1 restriction targets. (A) Target and masking determinants for Fv1^b (top) and Ref1 (bottom) restrictions in the MLV CA. Key residues involved in the susceptibility of N-MLV to Fv1^b and Ref1 restriction genes are circled (left panels, R110 and L117). The dashed circle indicates a fragment found to modulate the restriction level (see text). Residues sufficient to make the Friend MLV CA target inaccessible to Fv1^b and/or Ref1 restrictions are circled (right panels, combined D92 and H117 for Fv1^b; H117 for Ref1). (B) Three-dimensional structure of the amino-terminal domain of N-Akv CA (adapted from reference 21). Amino-terminal (position 1) and carboxy-terminal (position 131) extremities of the crystallized fragment are indicated as well as position 33 corresponding to the BamHI cloning site shown in Fig. 1 and α-helices 4, 5, and 6. Representation of asparagine 82 (N82), aspartate 92 (E92), arginine 110 (R110), and leucine 117 (L117) side chains highlights their potential accessibility to restriction factors.