Fig. 4. Analysis of SecA and of SecA derivatives by sucrose density centrifugation. The samples contained: (A) 9 µg of SecA; (B) 9 µg of SecA mixed with 0.15% ddm, run in a gradient containing 0.05% ddm; (C) 17 µg of N95-SecA; (D) 17 µg of N95-SecA-3Ala; (E) 17 µg of N95-SecA-6Ala. All samples also contained 50 µg of BSA. Samples (90 µl) from fractions 1–20 were analyzed by SDS–PAGE and stained with Coomassie Blue.