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A Phaseolin Domain Involved Directly in Trimer Assembly Is a Determinant for Binding by the Chaperone BiP

Ombretta Foresti, Lorenzo Frigerio, Heidi Holkeri, Maddalena de Virgilio, Stefano Vavassori and Alessandro Vitale
The Plant Cell
Vol. 15, No. 10 (Oct., 2003), pp. 2464-2475
Stable URL: http://www.jstor.org/stable/3871977
Page Count: 12
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
A Phaseolin Domain Involved Directly in Trimer Assembly Is a Determinant for Binding by the Chaperone BiP
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Abstract

The binding protein (BiP; a member of the heat-shock 70 family) is a major chaperone of the endoplasmic reticulum (ER). Interactions with BiP are believed to inhibit unproductive aggregation of newly synthesized secretory proteins during folding and assembly. In vitro, BiP has a preference for peptide sequences enriched in hydrophobic amino acids, which are expected to be exposed only in folding and assembly intermediates or in defective proteins. However, direct information regarding sequences recognized in vivo by BiP on real proteins is very limited. We have shown previously that newly synthesized monomers of the homotrimeric storage protein phaseolin associate with BiP and that phaseolin trimerization in the ER abolishes such interactions. Using different phaseolin constructs and green fluorescent protein (GFP) fusion proteins, we show here that one of the two α-helical regions of polypeptide contact in phaseolin trimers (35 amino acids located close to the C terminus and containing three potential BiP binding sites) effectively promotes BiP association with phaseolin and with secretory GFP fusions expressed in transgenic tobacco or in transfected protoplasts. We also show that overexpressed BiP transiently sequesters phaseolin polypeptides. We conclude that one of the regions of monomer contact is a BiP binding determinant and suggest that during the synthesis of phaseolin, the association with BiP and trimer formation are competing events. Finally, we show that the other, internal region of contact between monomers is necessary for phaseolin assembly in vivo and contains one potential BiP binding site.

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