cytoplasmic end of the channel thus making it inaccessible for even relatively small ions (e.g., iodide ion), to say nothing of proteases. The channel still remains closed also to the lumen if the polypeptide contains less than 70 amino acid residues (Crowley et al., 1994). The open state of the channel is induced upon interaction of its constituents with the nascent polypeptide chain of a sufficient length (more than 70 residues). Such construction of the channel and features of its interaction with the ribosome/ nascent chain complex predetermine the growing polypeptide chain to move unidirectionally through the channel towards the lumen of the endoplasmic reticulum.

In yeast, an additional mechanism of polypeptide chain translocation across the endoplasmic reticulum membrane has been revealed. In the case of translocation of some protein precursors (pre-proteins), their movement through the channel composed of the Sec61p heterotrimers is stimulated by special ATP-binding proteins (Kar2p or Lhs1p) on luminal side of the membrane, in combination with a transmembrane protein (Sec63p). In these cases a "driving force" for translocation is ensured by interaction of the trapped polypeptide chain with the Kar2p protein on the luminal end of the channel. This ATP-dependent step is controlled by the luminal DnaJ-like domain of protein Sec63p stimulating ATPase activity of Kar2p. Kar2p in its ADP-bound form associates with the incoming polypeptide chain, thus making translocation unidirectional. In other words, Kar2p in combination with Sec63p acts as a "molecular ratchet". Mutations in this protein disturbing its ability for ATP binding or interaction with Sec63p result in reduced cotranslational import of pre-proteins into the microsomal fraction of yeast cells. A similar role is probably played by Lhs1p, another yeast protein that also functions in combination with Sec63p. A Lhs1p homologue

Signal peptide + anchor (ST)

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