The endoplasmic reticulum ( ER) membrane protein ERj1, a member of the Hsp40 family, was proposed to be a regulator of protein biogenesis at the ER. With its lumenal J-domain, ERj1 recruits the lumenal Hsp70-type chaperone BiP to newly synthesized polypeptide chains. Its cytosolic domain interacts with ribosomes and inhibits protein synthesis in its BiP-free state. Additionally, the cytosolic domain may act as a transcription factor. Recent proteomic data suggest that ERj1 is a target of phosphorylation. Since protein kinase CK2 is present on the ER surface, we addressed the question whether ERj1 is a substrate for CK2. We show that native ERj1 is phosphorylated by CK2. Using deletion mutants, ERj1 peptides, and a mutational analysis, the major phosphorylation site for CK2 was mapped to the conserved sequence motif SSDEE at amino acid residues 477-481, which is located in the cytosolic domain of ERj1. (C) 2009 Elsevier Inc. All rights reserved.