Signal Sequences Initiate the Pathway of Maturation in the Endoplasmic Reticulum Lumen

Abstract

An interaction between an N-terminal signal sequence and the translocon leads to the initiation of protein translocation into the endoplasmic reticulum lumen. Subsequently, folding and modification of the substrate rapidly ensue. The close temporal coordination of these processes suggests that they may be structurally and functionally coordinated as well. Here we show that information encoded in the hydrophobic domain of a signal sequence influences the timing and efficiency of at least two steps in maturation, namely N-linked glycosylation and signal sequence cleavage. We demonstrate that these consequences correlate with and likely stem from the nature of the initial association made between the signal sequence and the translocon during the initiation of translocation. We propose a model by which these maturational events are controlled by the signal sequence-translocon interaction. Our work demonstrates that the pathway taken by a nascent chain through post-translational maturation depends on information encoded in its signal sequence.N-terminal signal sequences enable nascent secretory and transmembrane proteins to be targeted to the endoplasmic reticulum (ER) 1 for translocation. The signal sequence, newly emerged from the translating ribosome, is recognized in the cytoplasm by the signal recognition particle (1). By virtue of an interaction with its ER-localized receptor, signal recognition particle brings the ribosome-nascent chain complex to the ER membrane (2). Once at the ER, the ribosome is thought to facilitate the assembly or stabilization of the translocation channel, composed of the heterotrimeric Sec61 complex (3, 4). The 35-kDa polytopic Sec61␣ subunit appears to form the channel walls, whereas the smaller bitopic ␤-and ␥-subunits play an as yet undetermined role, perhaps in facilitating the insertion of the nascent chain into the channel (5-7).An important advance in the understanding of the initiation of translocation was the discovery of a second step of signal sequence recognition. In addition to being recognized by the signal recognition particle, the signal sequence also interacts with the translocation channel itself (8,9). Although the mechanism is not yet clear, this event is thought to stimulate the initiation of translocation. For the simplest secretory proteins, when the signal sequence is recognized by the channel, the ribosome assumes a tight interaction with the channel that shields the protein from the cytoplasm (8, 10). Concomitantly, the channel opens toward the ER lumen, either via a conformational change in the channel or removal of a molecular plug covering the luminal aperture (11,12).Not all signal sequences initiate translocation in the same way. Beyond merely stimulating the initiation of translocation, signal sequences can regulate the association between the ribosome and translocon and the exposure of the nascent chain to the cytoplasm or ER lumen (13-16). In at least one case, that of the prion protein, the consequence of this regulatory step is the governance of the ...

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