Detection of major Pr74Gag processing fragments by SDS-PAGE and Western blotting. Viral particles produced in HEK 293T cells were purified by ultracentrifugation through a 20% sucrose cushion and the pellets loaded on 15% gels. (A) Silver-stained SDS-PAGE. Lane 1: VLPs produced with oricoHERV-K113_GagProPol. Lane 2: VLPs produced by a mutant with an inactive protease (oricoHERV-K113_GagPro-Pol). Lane 3: Empty vector control. (B) Western blot analysis of the VLPs. Lane 1: oricoHERV-K113_GagProPol. Lane 2: oricoHERV-K113_GagPro-Pol (PR-mutant). The blots were probed using antisera generated against recombinant proteins of predicted MA (left panel), p15 (central panel) and CA (right panel) polypeptides of HERV-K113. The band marked with a star is an unspecific N-terminal degradation product of the Gag precursor that accumulates in the protease-deficient mutant. M, molecular mass marker.