Journal
JOURNAL OF CELL BIOLOGY
Volume 175, Issue 5, Pages 767-777Publisher
ROCKEFELLER UNIV PRESS
DOI: 10.1083/jcb.200608101
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- Telethon [GGP04129] Funding Source: Medline
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Although transmembrane proteins generally require membrane-embedded machinery for integration, a few can insert spontaneously into liposomes. Previously, we established that the tailanchored (TA) protein cytochrome b(5) (b5) can post-translationally translocate 28 residues downstream to its transmembrane domain (TMD) across protein-free bilayers (Brambillasca, S., M. Yabal, P. Sofientini, S. Stefanovic, M. Makarow, R. S. Hegde, and N. Borgese. 2005. EMBO J. 24: 2533 - 2542). In the present study, we investigated the limits of this unassisted translocation and report that surprisingly long ( 85 residues) domains of different sequence and charge placed downstream of b5's TMD can posttranslationally translocate into mammalian microsomes and liposomes at nanomolar nucleotide concentrations. Furthermore, integration of these constructs occurred in vivo in translocon-defective yeast strains. Unassisted translocation was not unique to b5 but was also observed for another TA protein ( protein tyrosine phosphatase 1B) whose TMD, like the one of b5, is only moderately hydrophobic. In contrast, more hydrophobic TMDs, like synaptobrevin's, were incapable of supporting unassisted integration, possibly because of their tendency to aggregate in aqueous solution. Our data resolve long-standing discrepancies on TA protein insertion and are relevant to membrane evolution, biogenesis, and physiology.
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