Aberrant activation of receptor tyrosine kinase (RTK) is usually a result of mutation and plays important roles in tumorigenesis. How RTK without mutation affects tumorigenesis remains incompletely understood. Here we show that in human melanomas pro-prion (pro-PrP) is an adaptor protein for an E3 ligase c-Cbl, enabling it to polyubiquitinate activated insulin-like growth factor-1 receptor (IGF-1R), leading to enhanced melanoma metastasis. All human melanoma cell lines studied here express pro-PrP, retaining its glycosylphosphatidy-linositol-peptide signal sequence (GPI-PSS). The sequence, PVILLISFLI in the GPI-PSS of pro-PrP, binds c-Cbl, docking c-Cbl to the inner cell membrane, forming a pro-PrP/c-Cbl/IGF-1R trimeric complex. Subse-quently, IGF-1R polyubiquitination and degradation are augmented, which increases autophagy and tumor metastasis. Importantly, the synthetic peptide PVILLISFLI disrupts the pro-PrP/c-Cbl/IGF-1R complex, reducing cancer cell autophagy and mitigating tumor aggressiveness in vitro and in vivo. Targeting cancer-associated GPI-PSS may provide a therapeutic approach for treating human cancers expressing pro-PrP.