The surface glycoprotein, Thy-1, when expressed by transfection in NG115/401L neural cells, inhibits their neurite outgrowth over astrocytes. We have investigated the role of the glycosylphosphatidylinositol anchor of Thy-1 in this inhibition. Hybrid molecules, in which the lipid anchor was replaced by polypeptide transmembrane domains, were expressed by transfection. Lines expressing Thy-1 with the transmembrane and full cytoplasmic domains of NCAM-140, or with the transmembrane and truncated cytoplasmic domain of CD8, were not inhibited in their ability to extend neurites over astrocytes. Truncation of the cytoplasmic domain of NCAM-140 to just two amino acids, however, produced a transmembrane form of Thy-1 that, when expressed at high levels, inhibited neurite outgrowth. All forms of Thy-1 were concentrated in clusters that occurred primarily on fine filopodia. In double transfectants expressing normal Thy-1 and Thy-1 with the full NCAM cytoplasmic tail, the clusters of each form were separate, with no instances of the transmembrane form being found within the clusters of lipid-anchored Thy-1. Thy-1 with the two-amino-acid cytoplasmic domain of NCAM also occurred in clusters separate from those occupied by lipid-anchored Thy-1, but substantial ‘invasion’ of the clusters of normal Thy-1 by this transmembrane construct occurred. We suggest that the ability of this hybrid protein to enter the lipid-anchored clusters enables it to activate the signalling pathways that normal Thy-1 uses. Thus the membrane anchor, in targetting Thy-1 to different microdomains on the cell surface, determines its ability to inhibit neurite outgrowth on astrocytes.

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