Conformation of polypyrimidine tract binding protein in solution.

The polypyrimidine tract binding protein (PTB) is an RNA binding protein that normally functions as a regulator of alternative splicing but can also be recruited to stimulate translation initiation by certain picornaviruses. High-resolution structures of the four RNA recognition motifs (RRMs) that make up PTB have previously been determined by NMR. Here, we have used small-angle X-ray scattering to determine the low-resolution structure of the entire protein. Scattering patterns from full-length PTB and deletion mutants containing all possible sequential combinations of the RRMs were collected. All constructs were found to be monomeric in solution. Ab initio analysis and rigid-body modeling utilizing the high-resolution models of the RRMs yielded a consistent low-resolution model of the spatial organization of domains in PTB. Domains 3 and 4 were found to be in close contact, whereas domains 2 and especially 1 had loose contacts with the rest of the protein.