Structural Characterization of Semen Coagulum-Derived SEM1(86–107) Amyloid Fibrils That Enhance HIV-1 Infection

SEM1(86-107) is a 22-residue peptide corresponding to residues 86-107 in the semenogelin I protein. SEM1(86-107) is an abundant component of freshly liquefied semen and forms amyloid fibrils capable of enhancing HIV infection. To probe the factors affecting fibril formation and gain a better understanding of how differences in pH between semen and vaginal fluid affect fibril stability, this study determined the effect of pH on SEM1(86-107) fibril formation and dissociation. The SEM1(86-107) fibril structure (i.e., residues that comprise the fibrillar core) was also probed using hydrogen-deuterium exchange mass spectrometry (HDXMS) and hydroxyl radical-mediated protein modification. The average percent exposure to hydroxyl radical-mediated modification in the SEM1(86-107) fibrils was determined without requiring tandem mass spectrometry spectral acquisition or complete separation of modified peptides. It was found that the residue exposures calculated from HDXMS and hydroxyl radical-mediated modification were similar. These techniques demonstrated that three regions of SEM1(86-107) comprise the amyloid fibril core and that positively charged residues are exposed, suggesting that electrostatic interactions between SEM1(86-107) and HIV or the cell surface may be responsible for mediating HIV infection enhancement by the SEM1(86-107) fibrils.