Identification of the poly-L-proline-binding site on human profilin

Profilin is a ubiquitous protein that has been implicated in the signaling pathway leading to cytoskeletal rearrangement in cells. An unusual property of profilin is its high binding affinity for poly-L-proline (PLP). This binding property is conserved in the profilins from diverse species with little sequence homology. We have monitored the binding of PLP to profilin by fluorescence and nuclear magnetic resonance spectroscopies. NMR spectroscopy has identified several residues whose amide nitrogen and amide hydrogen chemical shifts are significantly perturbed by binding of PLP. The affected residues are located at various locations throughout profilin's primary structure; however, mapping the location of the affected residues onto the recently determined three-dimensional solution structure of human profilin indicates that the effects of PLP binding are highly localized. Poly-L-proline binds profilin at the hydrophobic interface between profilin's NH2- and COOH-terminal helices and the upper face of its antiparallel beta-sheet. In contrast, residues located on the opposite side of the profilin structure are unaffected. The extent of the potential interaction surface of the PLP-profilin complex suggests that as few as 6 contiguous prolines would be sufficient for binding profilin. Examination of sequence data bases indicates that stretches of prolines of this length and longer occur in numerous regulatory proteins, suggesting that the ability of profilin to bind polyproline may be an important component of its signaling capabilities.