The central subunit of the protein translocation channel SecYE. Consists of two halves. 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 (By similarity). Component of the plastid Sec protein translocase complex, which is composed of at least SecY and SecE. Belongs to the SecY/SEC61-alpha family. Bioinformatics prediction programs detect only 8 instead of 10 transmembrane regions.