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. Found to be tightly associated with membrane-bound ribosomes, either directly or through adapter proteins (By similarity). Necessary for the biogenesis and transport of multi-pass membrane proteins into the ER membrane (By similarity). Plays a role in the pronephric kidney tubule development (PubMed:27392076). Heterotrimeric complex composed of SEC61-alpha, SEC61-beta and SEC61-gamma (By similarity). Component of the ribosome-associated ER translocon complex (By similarity). Localizes exclusively in granular structures in the endoplasmic reticulum (ER). Belongs to the SecY/SEC61-alpha family.