Fig 2. Heavy sedimenting structures are not bone fide polyribosomes.

Aliquots of concentrated polyribosomes were analysed by sedimentation velocity through 15–60% (w/v) sucrose density gradients. A) In the presence of MgCl₂, FMRP was detected at the level of polyribosomes (Poly) and the pellet fraction (Pel). After incubation with 30 mM EDTA (B) or treatment with 10 μg/ml RNase A (C), a clear displacement of the ribosomal L7 protein and FMRP towards the top of the gradient was observed, while both proteins were still detectable in the pellet (Pel). Conversely, in the presence of 0.4 M NaCl (D) or the anionic detergent deoxycholate (DOC, E), the majority of FMRP was found in the loading volume that did not penetrate the gradient, while the polyribosomal UV profile remained unchanged. In these conditions no UV-absorbing materials as well as no FMRP or L7 were detected in the pellet fraction.