Reorientation of the first signal-anchor sequence during potassium channel biogenesis at the Sec61 complex

摘要

The majority of the polytopic proteins that are synthesized at the ER (endoplasmic reticulum) are integrated co-translationally via the Sec61 translocon, which provides lateral access for their hydrophobic TMs (transmembrane regions) to the phospholipid bilayer. A prolonged association between TMs of the potassium channel subunit, TASK-1 [TWIK (tandem-pore weak inwardly rectifying potassium channel)-related acid-sensitive potassium channel 1], and the Sec61 complex suggests that the ER translocon co-ordinates the folding/assembly of the TMs present in the nascent chain. The N-terminus of both TASK-1 and Kcv (potassium channel protein of chlorella virus), another potassium channel subunit of viral origin, has access to the N-glycosylation machinery located in the ER lumen, indicating that the Sec61 complex can accommodate multiple arrangements/orientations of TMs within the nascent chain, both in?vitro and in?vivo . Hence the ER translocon can provide the ribosome-bound nascent chain with a dynamic environment in which it can explore a range of different conformations en route to its correct transmembrane topology and final native structure.Abbreviations: BMH, bismaleimidohexane; Cys-null, cysteine-null; EndoH, endoglycosidase H; ER, endoplasmic reticulum; Kcv, potassium channel protein of chlorella virus; OPG, opsin N-glycosylation tag; TASK-1, TWIK (tandem-pore weak inwardly rectifying potassium channel)-related acid-sensitive potassium channel 1; TM, transmembrane region; TRAM, translocating chain-associated membrane protein