Phosphorylation of serine 833 in cytoplasmic domain of low density lipoprotein receptor by a high molecular weight enzyme resembling casein kinase II

A soluble protein kinase that phosphorylates the last serine residue (Ser-833) in the cytoplasmic domain of the low density lipoprotein (LDL) receptor was purified about 1300-fold from the cytosol of bovine adrenal cortex. The LDL receptor kinase shared several properties with casein kinase II: 1) use of either GTP or ATP; 2) phosphorylation of a typical casein kinase II recognition sequence in the LDL receptor (a serine followed by a cluster of three negatively charged amino acids); and 3) inhibition by heparin. The LDL receptor kinase differed from classic casein kinase II in the following respects: 1) its apparent molecular weight on gel filtration was ~ 500,000 as opposed to the usual molecular weight of 130,000 for casein kinase II; 2) its affinity for the LDL receptor (apparent K(m) ~ 5 nM) was much greater than its affinity for casein (~ 10 μM); and 3) its activity was inhibited by polylysine, an agent that stimulates casein kinase II. The physiologic role of this unusual kinase, if any, is unknown.