Part of the Sec61 complex, which is the major component of a channel-forming translocon complex that mediates protein translocation across the endoplasmic reticulum (ER). The functional states of the translocon complex include co- and post-translational ER import, cotranslational membrane protein integration and retrograde transport of misfolded proteins out of the ER. In the cotranslational pathway, ribosomes synthesizing presecretory proteins are targeted to the translocon by the cytosolic signal recognition particle (SRP) and its ER-localized receptor. The association of the Sec61 complex with the ribosome is mediated by the 28S rRNA of the large ribosomal subunit. SRP-independent post-translational translocation requires the association of additional factors, such as the Sec62/63 complex and KAR2. In an initial step, the signal sequence seems to bind simultaneously to SEC61 and SEC62. A cycle of assembly and disassembly of Sec62/63 complex from SEC61 may govern the activity of the translocon. SEC61 mediates the association with the ribosome. Component of the heterotrimeric Sec61 complex, which is composed of SEC61, SBH1 and SSS1. Presumably three to four Sec61 heterotrimers assemble into an oligomeric ring with a central aqueous pore. In cotranslational ER import, the pore diameter varies from 9-15 A in a ribosome-free resting state to 40-60 A in a functional state when associated with the ribosome. The Sec61 complex is part of a channel-forming translocon complex whose composition seem to change dependent upon different functional states. During post-translational ER import the Sec61 complex associates with the Sec62/63 complex to form the Sec complex. SEC61 interacts with STT3, OST1, OST2, OST4, SWP1 AND WBP1 components of the OT complex. Present with 24800 molecules/cell in log phase SD medium. Belongs to the SecY/SEC61-alpha family.