The aggregation tendencies of the signal peptide regions of prone and not prone to aggregate proteins

Signal peptides are a sequence of peptides located at the N-terminus of proteins and determine the protein secretion pathway and their destinations. The N-region is positively charged and influences the orientation of translocation. When linked to the protein, signal peptides provide protection against misfolding events and the formation of amyloid fibrils, which are related to neurodegenerative and inflammatory diseases. An earlier study from our research group demonstrated the tendency/predisposition of misfolding and seeding events of the synthetic signal peptide of serum amyloid A1 (SAA1), which originates from humans and other animal species. The current study explores the propensity for misfolding of fourteen different synthetic signal peptides of both prone-to-aggregate and non-prone-to-aggregate proteins by an in silico program, Congo red (CR) binding assays, and transmission electron microscopy (TEM). Among the six signal peptides derived from aggregation-prone proteins, all of them were confirmed to misfold by electron microscopy and CR binding assays. More precisely, the signal peptides from the human and feline islet amyloid polypeptide (IAPP), SAA1, parathyroid hormone (PTH), transthyretin (TTR), and cathepsin K (CTSK) generated amyloid-like fibrils. Signal peptides from the non-prone-to-aggregate protein category were capable of misfolding, such as human collagen type X alpha 1 (COL10A1), granulin (GRN), luteinizing hormone beta polypeptide (LHB), serpin peptidase inhibitor A7 (SERPINA7), serpin peptidase inhibitor E (SERPINE1), serine protease 1 (PRSS1), and UDP glucuronosyltransferase family 1 member A1 signal peptides (UGT1A1). TTR signal peptide seeds accelerated the misfolding of TTR1-25 fragment peptide -which was assessed by thioflavin T assays- and increased the formation of oligomers as validated by the photo-induced cross-linking of unmodified proteins assays. Overall, all the prone-to-aggregate signal peptides and seven out of the eight non-prone-to-aggregate signal peptides were confirmed to misfold as shown by TEM images. Studying the tendency of signal peptides to aggregate may provide key indicators into the mechanisms underlying protein misfolding.