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 phosphorylated and non-phosphorylated forms of neurofilament (NF)-H. N52 is therefore classified as a phosphorylation-independent-type antibody. 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 reactivity. A fragment of NF-H containing the consensus cdk-5 sites was reactive 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-independent-type NF-H antibody and suggest that the N52 epitope contains sites targeted for phosphorylation by cdk-5.