Fig 4.

Plasmid-encoded gp20 is found in the membrane fraction, whereas gp40 is in the supernatant. E. coli BL21 cultures expressing His-gp20 with gp40 and His-gp20s with gp40 were lysed by a French press and analyzed for soluble and membrane-bound protein (A). The soluble fraction (S) and the membrane fraction (P) were collected by centrifugation and analyzed for their protein content on a Western blot using the respective antibodies. (B) The membrane fraction was placed onto a sucrose step gradient, and fractions 1 to 8 were analyzed for their content of leader peptidase (inner membrane marker protein, top panel), His-gp20 (central panel), and His-gp20s (lower panel) on Western blots.