Sec61 is an endoplasmic reticulum (ER) membrane protein translocator (aka translocon). It is a doughnut shaped pore through the membrane with 3 major subunits (heterotrimeric). It has a region called the plug that blocks transport into or out of the ER. This plug is displaced when the hydrophobic region of a nascent polypeptide interacts with another region of Sec61 called the seam, allowing translocation of the polypeptide into the ER lumen.
Read more about Sec61: Human Proteins