Abstract
Sec61p is the channel-forming subunit of the heterotrimeric Sec61 complex that mediates co-translational protein import into the endoplasmic reticulum (ER). In yeast, proteins can also be post-translationally translocated by the hetero-heptameric Sec complex, composed of the Sec61 and the Sec63 complexes. The Sec61 channel is also a candidate for the dislocation channel for misfolded proteins from the ER to the cytosol during ER-associated degradation (ERAD). The structure of the Sec61 complex is highly conserved, but the roles of its N-terminal acetylation and its amphipathic N-terminal helix are unknown so far. To gain insight into the function of the Sec61p N-terminus, we mutated its N-acetylation site, deleted its amphipathic helix, or both the helix and the N-acetylation site. Mutation of the N-acetylation site on its own had no effect on protein import into the ER in intact cells, but resulted in an ERAD defect. Yeast expressing sec61 without the N-terminal amphipathic helix displayed severe growth defects and had profound defects in post-translational protein import into the ER. Nevertheless the formation of the hetero-heptameric Sec complex was not affected. Instead, the lack of the N-terminal amphipathic helix compromised the integrity of the heterotrimeric Sec61 complex. We conclude that the N-terminal helix of Sec61p is required for post-translational protein import into the ER and Sec61 complex stability, whereas N-terminal acetylation of Sec61p plays a role in ERAD.
Highlights
Secretory proteins and organelle proteins of the secretory pathway are translocated into the endoplasmic reticulum (ER) during biogenesis [1]
Proteins that fail to fold in the ER are retrotranslocated to the cytosol in order to be degraded by proteasomes, a process known as ER-associated degradation (ERAD) [2, 3]
We have shown here that N-acetylation site of Sec61p at S2 is important for ERAD (Fig 2A), and may contribute to post-translational import into the ER (Fig 4A), whereas its N-terminal amphipathic helix is essential for post-translational import into the ER and is required for stability of the Sec61 complex (Fig 3C, Fig 5B)
Summary
Secretory proteins and organelle proteins of the secretory pathway are translocated into the endoplasmic reticulum (ER) during biogenesis [1]. In the ER lumen, imported proteins have to acquire a functional conformation before their delivery to specific cellular destinations via the secretory pathway [2]. Transport of newly synthesized proteins across the ER membrane can occur either co- or post-translationally [4]. Both modes of translocation require the heterotrimeric Sec channel, which consists of three proteins, Sec61p, Sbh1p, and Sss1p in yeast (Sec61α, β, γ in mammals) [5]. The Sec complex is sufficient to mediate co-translational import on its own, while it associates with the heterotetrameric Sec complex (Sec62p, Sec63p, Sec71p, Sec72p) for post-translational protein import into the yeast ER [5].
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