Evidence for Multiple Mechanisms for Membrane Binding and Integration via Carboxyl-Terminal Insertion Sequences

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• 作者：Peter K. Kim ; Fabiola Janiak-Spens ; William S. Trimble ; Brian Leber ; and David W. Andrews
• 刊名：Biochemistry
• 出版年：1997
• 出版时间：July 22, 1997
• 年：1997
• 卷：36
• 期：29
• 页码：8873 - 8882
• 全文大小：248K
• 年卷期：v.36,no.29(July 22, 1997)
• ISSN：1520-4995

文摘

Subcellular localization of proteins withcarboxyl-terminal insertion sequences requires themolecule be both targeted to and integrated into the correct membrane.The mechanism of membraneintegration of cytochrome b₅ has been shown tobe promiscuous, spontaneous, nonsaturable, and independentof membrane proteins. Thus endoplasmic reticulum localization forcytochrome b₅ depends primarily onaccurate targeting to the appropriate membrane. Here directcomparison of this mechanism with that ofthree other proteins integrated into membranes via carboxyl-terminalinsertion sequences [vesicle-associatedmembrane protein 1(Vamp1), polyomavirus middle-T antigen, and Bcl-2]revealed that, unlike cytochromeb₅, membrane selectivity for these molecules isconferred at least in part by the mechanisms of membraneintegration. Bcl-2 membrane integration was similar to that ofcytochrome b₅ except that insertionintolipid vesicles was inefficient. Unlike cytochromeb₅ and Bcl-2, Vamp1 binding to caninepancreaticmicrosomes was saturable, ATP-dependent, and abolished by mild trypsintreatment of microsomes.Surprisingly, although the insertion sequence of polyomavirusmiddle-T antigen was sufficient to mediateelectrostatic binding to membranes, binding did not lead to integrationinto the bilayer. Together theseresults demonstrate that there are at least two different mechanismsfor correct membrane integration ofproteins with insertion sequences, one mediated primarily by targetingand one relying on factors in thetarget membrane to mediate selective integration. Our results alsodemonstrate that, contrary to expectation,hydrophobicity is not sufficient for insertion sequence-mediatedmembrane integration. We suggest thatthe structure of the insertion sequence determines whether or notspecific membrane-bound receptor proteinsare required for membrane integration.