The central subunit of the protein translocation channel SecYEG. Consists of two halves formed by TMs 1-5 and 6-10. These two domains form a lateral gate at the front which open onto the bilayer between TMs 2 and 7, and are clamped together by SecE at the back. The channel is closed by both a pore ring composed of hydrophobic SecY resides and a short helix (helix 2A) on the extracellular side of the membrane which forms a plug. The plug probably moves laterally to allow the channel to open. The ring and the pore may move independently. Complements an E.coli temperature-sensitive secY mutation; deletion of the last 15 residues prevents complementation, which may indicate a role of this region in translocation. Component of the Sec protein translocase complex. Heterotrimer consisting of alpha (SecY), beta (SecG) and gamma (SecE) subunits. The heterotrimers can form oligomers, although 1 heterotrimer is thought to be able to translocate proteins. Interacts with the ribosome. May interact with SecDF, and other proteins may be involved. The beta subunit (SecG) was lost during crystallization. Belongs to the SecY/SEC61-alpha family.