GRP94 (endoplasmin) co-purifies with and is phosphorylated by Golgi apparatus casein kinase

A phosphorylatable protein band of about 94 kDa (as judged by SDS-PAGE) which co-purifies and co-immunoprecipitates with Golgi apparatus casein kinase (C-CK) from rat lactating mammary gland has been shown bg mass spectrometric sequence analysis to be identical or very similar to the glucose-regulated protein, GRP94, GRP94 is also readily phosphorylated by G-CK (K-m = 0.2 mu M) at seryl sites which are different from the sites affected by casein kinase-2 (CK2) in the same protein. A study with peptide substrates would indicate that the G-CK sites in GRP94 conform to the motif S-R/K-E-X (X being different from D and E) which is not recognized by CK2. (C) 2000 Federation of European Biochemical Societies.