Abstract
: The neural cell adhesion molecule LI is a phos‐phorylated integral membrane glycoprotein that is recovered from adult mouse brain by immunoaffinity chromatography as a set of polypeptides with apparent molecular masses of 200, 180, 140, 80, and 50 kilodaltons (LI‐200, LI‐180, Ll‐140, LI‐80, and LI‐50, respectively). In the present study, we show that two kinase activities are associated with im‐munopurified L1: One specifically phosphorylates L1‐200 and L1‐80 but not L1‐180, L1‐140, or L1‐50. This pattern of hosphorylation corresponds to the one described for L1 after metabolic phosphate incorporation into cultures of cerebellar cells. In both cases, serine is the main amino acid that is labeled by radioactive phosphate. The kinase activity is not activated by Ca2+, calmodulin, phosphatidylserine, diolein, cyclic AMP, or cyclic GMP, a result suggesting that the enzyme is distinct from Ca2+/calmodulin‐dependent kinases, from protein kinase C, or from cyclic AMP/cyclic GMP‐dependent kinases and may belong to the independent kinase group. The other kinase phosphorylates only casein but not L1, utilizes GTP as well as ATP, and is strongly inhibited by heparin. Because the primary structure of the L1 protein does not contain consensus sequences characteristic for known kinases, we believe that the catalytic activities detectable in immunopurified L1 are due to kinases that are strongly enough associated with L1 to withstand the stringent purification procedures.