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First published online 13 May 2008
doi: 10.1242/jcs.020321

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Distinct targeting pathways for the membrane insertion of tail-anchored (TA) proteins

Vincenzo Favaloro^*, Milan Spasic^*,, Blanche Schwappach and Bernhard Dobberstein^¶

Zentrum für Molekulare Biologie der Universität Heidelberg (ZMBH), DKFZ-ZMBH Allianz, Im Neuenheimer Feld 282, D-69120 Heidelberg, Germany

^¶ Author for correspondence: (e-mail: b.dobberstein{at}zmbh.uni-heidelberg.de)

Accepted 2 March 2008

Tail-anchored (TA) proteins are characterised by a C-terminal transmembrane region that mediates post-translational insertion into the membrane of the endoplasmic reticulum (ER). We have investigated the requirements for membrane insertion of three TA proteins, RAMP4, Sec61β and cytocrome b5. We show here that newly synthesised RAMP4 and Sec61β can accumulate in a cytosolic, soluble complex with the ATPase Asna1 before insertion into ER-derived membranes. Membrane insertion of these TA proteins is stimulated by ATP, sensitive to redox conditions and blocked by alkylation of SH groups by N-ethylmaleimide (NEM). By contrast, membrane insertion of cytochrome b5 is not found to be mediated by Asna1, not stimulated by ATP and not affected by NEM or an oxidative environment. The Asna1-mediated pathway of membrane insertion of RAMP4 and Sec61β may relate to functions of these proteins in the ER stress response.

Key words: Tail-anchored protein, RAMP4, Sec61β, Cytochrome b5, Endoplasmic reticulum, Asna-1/TRC40