CELLULAR PHOSPHORYLATION OF NEUROFILAMENT HEAVY-CHAIN BY CYCLIN-DEPENDENT KINASE-5 MASKS THE EPITOPE FOR MONOCLONAL-ANTIBODY N52

N52 is a widely used monoclonal antibody reported to recognise both ph osphorylated and non-phosphorylated forms of neurofilament (NF)-H. N52 is therefore classified as a phosphorylation-independent-type antibod y. N52 is strongly reactive with NF-H in COS cells transfected with NF -H alone but co-transfection of NF-H with the neurofilament kinase cdk -5 and one of its activators p35, induced phosphorylation of NF-H that abolished this reactivity. Treatment of the cdk-5 phosphorylated NF-H with alkaline phosphatase so as to remove phosphate restored N52 reac tivity. A fragment of NF-H containing the consensus cdk-5 sites was re active with N52 but following co-transfection with cdk-5/p35 a slower migrating fragment species generated by cdk-5 was not labelled by N52. These results demonstrate that N52 is not a truly phosphorylation-ind ependent-type NF-H antibody and suggest that the N52 epitope contains sites targeted for phosphorylation by cdk-5.