Plays a crucial role in the insertion of secretory and membrane polypeptides into the ER. Required for assembly of membrane and secretory proteins. Tightly associated with membrane-bound ribosomes, either directly or through adapter proteins (By similarity). Plays a role in pronephric kidney tubule development (By similarity). Heterotrimeric complex composed of SEC61-alpha, SEC61-beta and SEC61-gamma. Localizes exclusively in granular structures in the endoplasmic reticulum (ER). Belongs to the SecY/SEC61-alpha family.