We have analyzed a series of Moloney murine leukemia (M-MuLV) envelope (env) protein fusions to the marker proteins invertase and placental alkaline phosphatase (PLAP), expressed in Psi2 retrovirus packaging cells. he yeast invertase protein, fused at its third amino acid residue to the amino-terminal signal sequence and 17 residues of the mature M-MuLV env protein, retained its enzymatic activity and was secreted from mammalian cells. However, env protein fusions to the C-terminal portion of invertase were inactive. In contrast, some, but not all, env protein fusions at the C-terminal region of PLAP were enzymatically active: PLAP fusions containing long C-terminal portions of env localized to the rough endoplasmic reticulum (RER) and possessed low enzyme activity levels, while fusion constructs containing relatively short portions of the M-MuLV env gene localized to the Golgi and had higher activity levels. Those proteins that localized to the Golgi also were processed, in part, to forms of 67 to 68 kDa, the size of the mature PLAP protein. Since PLAP ordinarily is transfered to a phosphatidyl-inositol glycan tail (PIG-tail) in the Golgi and then transported to the plasma membrane, it appears that Golgi-localized PLAP-env fusions are processed imperfectly. PLAP itself, when expressed in Psi2 cells, accumulated at the plasma membrane and, unlike the PIG-tailed Thy-1 protein, was not incorporated into virus particles. Thus, the reported incorporation of the Thy-1 protein into M-MuLV virions does not appear to be a consequence of its glycoprotein tail.
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