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Biochem Biophys Res Commun. 2002 Mar 22;292(1):153-60.

Membrane topogenesis of the three amino-terminal transmembrane segments of glucose-6-phosphatase on endoplasmic reticulum.

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Department of Molecular Biology, Department of Clinical Chemistry and Laboratory Medicine, Graduate School of Medical Science, Kyushu University, Maidashi 3-1-1, Higashiku, Fukuoka 812-8582, Japan.


We investigated the membrane topogenesis of glucose-6-phosphatase (G6Pase), a multispanning membrane protein, on the endoplasmic reticulum. In COS-7 cells, the first transmembrane segment (TM1) with weak hydrophobicity is inserted into the membrane in the N-terminus-out/C-terminus-cytoplasm orientation. The following TM2 is inserted depending on TM3. TM3 has the same orientation as TM1. In contrast to data from living cells, the full-length molecule and N-terminal fusion constructs were not inserted into the membrane in a cell-free system. Addition of a signal recognition particle did not improve G6Pase insertion. When the 37-residue N-terminal segment was deleted, however, TM2 and TM3 were correctly inserted. We concluded that the three N-terminal TM segments are inserted into the membrane dependent on the two signal-anchor sequences of TM1 and TM3. TM1 is likely to be an unconventional signal sequence that barely functions in vitro. The 37-residue N-terminal segment inhibits the signal function of the following TM3 in cell-free systems.

[Indexed for MEDLINE]

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