Polyadenine complexed to polyglutamine suggests the peptide backbone has a cis conformation

The polyadenine (poly(A)) tail of mRNA is a homopolymer and as such is a potential H-bonding partner for other cellular homopolymers. The secondary structure of proteins and peptides employs the polar groups of the homopolymer backbone to bind with themselves or other polymers to respectively form an alpha helix or a beta sheet. These same backbone polar groups appear to be suitably positioned to bind with poly(A) but only when the backbone has rotated to the all-cis conformation. However the all-cis conformation of peptides or proteins is rarely encountered and so they would seem to be unlikely binding partners. Here we show that a homopolymer of glutamine (poly(Q)) may be an exception to this rarity because its default conformation seems to be all-cis. We found that exposure of poly(A) to increasing amounts of poly(Q) causes a progressive loss of soluble poly(A) migrating on gel electrophoresis, indicating binding between the two polymers. Stereochemical modelling of the likely complex has suggested the particular polar groups that are responsible for binding of the two polymers, along with an explanation for the all-cis conformation of poly(Q).