Membrane-bound polyribosomes in HeLa cells: association of polyadenylic acid with membranes

Polyadenylic acid of membrane-bound polyribosomes is shown to be associated with rapidly sedimenting membrane structures. Most of this poly(A) remains attached to membranes after extensive degradation of polyribosomal messenger RNA with pancreatic ribonuclease. Previously, it was shown that exposure to EDTA removes up to 40% of the membrane-associated mRNA. In our experiments, 57% of the membrane-associated poly(A) still sediments with membrane structures after treatment with pancreatic ribonuclease followed by the addition of EDTA. This indicates that the association of about 60% of the membrane-bound poly(A) is EDTA-resistant, while the remainder is labile after removal of magnesium ions. Reconstruction experiments suggest that poly(A) from detergent-treated, membrane-derived polyribosomes is not trapped by other membrane structures. The poly(A)-containing RNA fragment that remains associated with the membranes after pancreatic ribonuclease treatment is shown to be a single peak at about 7 S, or about the size of cellular poly(A). Thus, the attachment site is almost pure poly(A). The poly(A)-containing, membrane-bound mRNA appears to be of a larger average size than total cellular poly(A)-containing RNA, as judged by its greater sedimentation value.