Host−Guest Study of Left-Handed Polyproline II Helix Formation

Abstract

The importance of the left-handed polyproline II (PPII) helical conformation has recently become apparent. This conformation generally is involved in two important functions: protein-protein interactions and structural integrity. PPII helices play vital roles in a variety of processes including signal transduction, transcription, and cell motility. Proline-rich regions of sequence are often assumed to adopt this structure. Remarkably, little is known about the physical determinants of this secondary structure type. In this study, we have explored the formation of PPII helices by a short poly(proline) peptide. In addition, the results from experiments used to determine the propensities for apolar residues, plus glycine, asparagine, and glutamine, to adopt this structure in a poly(proline)-based host peptide are reported here. Proline possesses the highest intrinsic propensity, with glutamine, alanine, and glycine having surprisingly high propensities.-Branched residues possess the lowest propensities of the residues examined. It is postulated that propensities possessed by apolar residues are due in part to peptide-solvent interactions, and that the remarkably high propensity possessed by glutamine may be due to a side chain to backbone hydrogen bond. These data are the first step toward a molecular understanding of the formation of this important, and yet little studied, secondary structure.In recent years, the left-handed polyproline II (PPII) 1 helical conformation has been elevated from the status of a relatively rare and seemingly uninteresting secondary structure to one that is surprisingly common and of the utmost importance. This structure plays a central role in numerous vital processes including signal transduction, transcription, cell motility, and the immune response. Proline-rich ligands of the cytoskeletal protein profilin (1), as well as those of the SH3, WW, and EVH1 protein interaction domains, are bound in this conformation (2). The peptide ligands of class II MHC molecules are also bound in the PPII conformation (3). PPII helices are major features of collagens (4) and plant cell wall proteins (5). The PPII helix is believed to be the dominant conformation for many proline-rich regions of sequence (PRRs) (6). Sequences not rich in proline also adopt this structure. For example, poly(lysine), poly(glutamate), and poly(aspartate) peptides form PPII helices (7). Around 2% of all residues in known protein structures are found in PPII helices at least four residues long (8, 9). As many as 10% of all residues are found in the PPII conformation, although not necessarily as part of PPII helices (10). PPII helices have also been hypothesized to be a major component of protein denatured states (11-14), giving them a role in a most fundamental process. Recently, Blanch et al. (15) have suggested that the PPII helix might be the precursor conformation in amyloid formation. Given the preceding, it is truly remarkable how little is known about the physical determinants of the PPII helical conformat...

CD80 Binding Polyproline Helical Peptide Inhibits T Cell Activation

“…Studies of the synthetic peptide made up of MYPPPY sequence do not exhibit PP II helical conformation and had no inhibitory potential in cellular assays (35). This may be attributed to the lower potential of aromatic amino acids to propagate PP II helix (36). Integrating the residue preferences and propensities, novel CD80-CAP hexapeptides were designed so as to possess significant PP II helical content in the context of the CD80 binding interface.…”

Three intrinsically unstructured mussel adhesive proteins, mfp‐1, mfp‐2, and mfp‐3: Analysis by circular dichroism

“…The PPII content in mfp-1 decreased with increasing temperature. Using a PP II prediction model based on short polyproline peptides, 25 PP II structure reduction in mfp-1 with increasing temperature was observed but it is slight at best (from $54% at 4 C to 51% at 28 C). pH effects on the CD spectrum of mfp-1 were also investigated between Fig.…”

“…Estimates of PPII helix content in mfp-1 were obtained by using the following equation. 25,42,43 %PPII ¼ ½h max þ 6700 13; 700 Â 100 where [y] max is the molar ellipticity at the characteristic maximum, À6100 deg dmol À1 cm 2 is taken as the lower limit ($0% PPII), and 7600 deg dmol À1 cm 2 is taken as the upper limit (PPII, $100%).…”

Phosphorylation as Conformational Switch from the Native to Amyloid State: Trp-Cage as a Protein Aggregation Model

“…This spectrum resembles to the spectra of polyproline II helices, proline-rich peptides, or left-handed triple helices of collagen. 50,51 This finding is further supported by the fact that the C-terminal half of the Trp-cage is rich in proline residues.…”