Appears to play a crucial role in the insertion of secretory and membrane polypeptides into the ER. It is required for assembly of membrane and secretory proteins and is essential for cell growth. It interacts with other membrane proteins required for protein translocation. Upon binding to SEC62/63 complex, secretory precursor polypeptides may engage SEC61 to begin membrane penetration event. A cycle of assembly and disassembly of SEC62/63 from SEC61 may govern the activity of the translocase (By similarity). Heterotrimeric complex composed of SEC61-alpha, SEC61-beta and SEC61-gamma. Belongs to the SecY/SEC61-alpha family.