Phosphatidylglycerol participates in syncytium formation induced by HTLV type 1-bearing cells

We previously reported that 71-kDa heat shock cognate protein (HSC70) was e xpressed on the cell surface of human T cell lymphotropic virus type 1 (HTL V-1)-susceptible cells and that HSC70, beta -actin, and a lipid-like compon ent on the target cell membrane participated in syncytium formation by HTLV -1. We have now identified this lipid-like component to be palmitoyl (16:0) -oleoyl (18:1)-phosphatidylglycerol (POPG), using preparative thin-layer ch romatographic fractionation and tandem mass spectrometric analysis. In the syncytium formation-assay, exogenously added PG inhibited cell-to-cell tran smission of HTLV-1 in a dose-dependent manner. Other phospholipids showed l ess (PE) or no effect (PC, PS, PI, PA, lysoPC, lysoPE, and CL). Binding exp eriments showed that PG interacted with three synthetic peptides, gp46-111, gp46-197, and gp21-400, which correspond to regions Lys111-Asp138 and Asp1 97-Leu216 on the gp46 surface glycoprotein, and to region Cys400-Leu429 on the gp21 transmembrane glycoprotein, respectively, as well as with intact g p46 and gp21 proteins of HTLV-1. On the other hand, HSC70 and beta -actin i nteracted with gp46-197 and gp46, not with gp46-111. However, the eluate fr om an affinity column coupled with gp46-111 contained not only PG but also HSC70 and beta -actin, despite the lack of direct interaction between gp46- 111 and these proteins. In the in vitro binding assay, HSC70 showed interac tion with both PG and beta -actin, while there was no evidence of any inter action between PG and beta -actin. These results suggest that HSC70 molecul es on target cell surface interact with both PG in lipid bilayers and intra cellular beta -actin and that these three cellular components form a recept or complex that plays a critical role in syncytium formation induced by HTL V-1-bearing cells.