Glycoprotein incorporation and Gag processing. COS-1 cells were transfected with nothing (lane 1), pSAMR4 (lane 2), ΔKKPKR (lane 3), or pMT ΔE (lane 4) and then labeled overnight with [3H] leucine. Culture medium was filtered and viral particles were pelleted through a 20% sucrose cushion. The viral proteins present in the pellet were immunoprecipitated using goat anti-M-PMV antibodies. Positions of various viral proteins are indicated.