Molecular signals for phosphatidylinositol modification of the Qa-2 antigen.
Most cell surface proteins are anchored to the cell bilayer by hydrophobic membrane-spanning domains. Recently it has been shown that a small class of molecules are attached to cell surfaces via a phosphatidylinositol moiety covalently linked to the C-terminus of the mature processed polypeptide. The molecular signals that identify a polypeptide for phosphatidylinositol (PI) attachment have not been well defined in any system, but are thought to reside in the C-terminus of the primary translation product. We report that all the signals responsible for PI anchoring of Qa-2 Ag are confined to the 36 C-terminal residues of the precursor proteins. To investigate further the features that signal cleavage and PI addition, we have studied mutants of two closely related murine class I MHC molecules: the PI-linked Ag, Q9b, from the Qa-2 Ag family, and the integral membrane transplantation antigen, H-2Ld. The addition of 15 amino acids to the three residue long cytoplasmic domain of Q9b or the mutation of Asp295 found in its C-terminal hydrophobic domain to Val converts this molecule into an integral membrane protein. However, the introduction of a short three residue cytoplasmic tail and Asp295 into the transmembrane domain of H-2Ld does not convert this molecule to a PI-linked one. The results of these analyses suggest that the PI-processing signals may depend on overall conformation, hydrophobicity, and length of the C-terminal domain of the precursor protein. In addition these data indicate that PI anchoring of class I Ag requires more than two mutational steps and may have been selected during the evolution.
Ulker
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Division of Biology, California Institute of Technology, Pasadena 91125.
Hood
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Stroynowski
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D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D004252:DNA Mutational Analysis; D006183:H-2 Antigens / Q000235:genetics*; D015395:Histocompatibility Antigens Class I / Q000235:genetics / Q000378:metabolism*; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D051379:Mice; D008969:Molecular Sequence Data; D009841:Oligonucleotides; D010716:Phosphatidylinositols / Q000378:metabolism*; D011499:Protein Processing, Post-Translational; D013329:Structure-Activity Relationship; D014162:Transfection
D006183:H-2 Antigens; D015395:Histocompatibility Antigens Class I; D008565:Membrane Proteins; D009841:Oligonucleotides; D010716:Phosphatidylinositols; C032907:Q surface antigens