However, Olmesartan medoxomil
by CK2 may influence ERj1 function besides the ribosome interaction. Several components of the protein transport machinery were found to be phosphorylated. In addition to calnexin, SRα, the α-subunit of the SRP receptor was shown to be a substrate of the protein kinase CK2 . The role of the SRα phosphorylation is still unknown. Interestingly, also the two components of the transport machinery Sec61β and the TRAM protein are phosphoproteins. Both are phosphorylated by the Ca2+-dependent kinase PKC and phosphorylation results in higher translocation efficiency . A further hint for the importance of CK2-dependent phosphorylation of components of the transport machinery comes from studies in the yeast Saccharomyces cerevisiae. The essential Sec63p chaperone, another ER-resident member of the Hsp40 family, is phosphorylated by the protein kinase CK2 as well. Nonphosphorylatable Sec63p causes a protein translocation defect . So far, it is not clear whether the Sec63 mammalian homolog is also phosphorylated by CK2. It is interesting to note that ERj1, which has no homolog in yeast, can rescue the thermosensitive phenotype
of a SEC63 translocation-deficient mutant in yeast . Taken together, these findings suggest a general role of phosphorylation for a network of transport factors in regulation of protein translocation across the ER-membrane.