Mediates cotranslational and post-translational transport of certain precursor polypeptides across endoplasmic reticulum (ER) (, ). Proposed to play an auxiliary role in recognition of precursors with short and apolar signal peptides. May cooperate with SEC62 and HSPA5/BiP to facilitate targeting of small presecretory proteins into the SEC61 channel-forming translocon complex, triggering channel opening for polypeptide translocation to the ER lumen . Required for efficient PKD1/Polycystin-1 biogenesis and trafficking to the plasma membrane of the primary cilia (By similarity).
Subcellular locations: Endoplasmic reticulum membrane
Widely expressed, with high levels in the liver.