Changing the spacing between metal-binding motifs decreases stability and transforming activity of the human papillomavirus type 18 E7 oncoprotein

Abstract

Spacing composed of 29 amino acids (AAs) between a pair of metal-binding motifs, Cys-X-X-Cys, is common to human papillomavirus (HPV) zinc-binding oncoproteins E6 and E7 from various HPV types. From the HPV 18 E7 gene encoding a 105-AA protein with the motifs at AAs 65-68 and 98-101, we constructed expression plasmids for two mutants, 18del73 and 18ins84, with varied spacing between the motifs. Mutant proteins 18del73 and 18ins84 had a deletion from AAs 74 to 88 and an insertion of three AAs substituting for AA 85, respectively. The mutations lowered the efficiency of E7-mediated focal transformation of rat 3Y1 cells, approximately parallel to the reduced level of steady-state E7 expressed in COS-1 cells. It is likely that, besides the presence of the motifs, the conserved spacing between the motifs is important for a stable structure of E7, but is not essential for the E7-transforming activity.