The abnormal appearance and age-dependent loss of resident fibroblast growth factor receptor-2 (FGFR2) and gain of activity of FGFR1 in epithelial cells is a hallmark of the slow progression to malignancy in some models of prostate cancer. Pericellular matrix heparan sulfate (HS) is an integral subunit of the FGFR tyrosine kinase complex that restricts activity in absence of FGF, facilitates binding of an activating FGF, and confers specificity for FGF isoforms. In this report, we isolated and purified HS proteoglycan (HSPG) from premalignant prostate tumor epithelial cells based on the ability of the HS chains to form a binary complex with immunoglobulin module II of the ectopic and progression-promoting FGFR1 that was competent to bind FGF. The FGFR1 affinity-purified product exhibited a specific activity of over 600 times that of crude cellular HSPG enriched from cell lysates by ion exchange chromatography. The purified preparation exhibited a single NH(2)-terminal sequence with 11 of 13 residues identical to syndecan-1. The activity of purified recombinant glutathione S-transferase-tagged syndecan-1 expressed in premalignant epithelial cells confirmed that syndecan-1 bears HS chains that exhibit the rare motif that forms the FGF-binding complex with ectopic FGFR1. These results are the first to identify by affinity purification a specific HSPG core protein, the HS chains of which act as an integral subunit of the FGFR complex. The results suggest that syndecan-1 provides HS chains in premalignant epithelial cells to both the FGFR2- and FGFR1-signaling complexes that are integral to their dual roles in progression to malignancy.