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Endogenously expressed Ranbp2 is not at the axon initial segment

ABSTRACT Ranbp2 (also known as Nup358) is a member of the nucleoporin family, which constitutes the nuclear pore complex. Ranbp2 localizes at the nuclear membrane and was recently reported at the axon initial segment (AIS). However, we show that the anti-Ranbp2 antibody used in previous studies is not specific for Ranbp2. We mapped the antibody binding site to the amino acid sequence KPLQG, which is present in both Ranbp2 and neurofascin (Nfasc), a well-known AIS protein. After silencing neurofascin expression in neurons, the AIS was not stained by the antibody. Surprisingly, an exogenously expressed N-terminal fragment of Ranbp2 localizes at the AIS. We show that this fragment interacts with stable microtubules. Finally, using CRISPR/Cas9 in primary cultured neurons, we inserted an HA-epitope tag at N-terminal, C-terminal or internal sites of the endogenously expressed Ranbp2. No matter the location of the HA-epitope, endogenous Ranbp2 was found at the nuclear membrane but not the AIS. These results show that endogenously expressed Ranbp2 is not found at AISs. This article has an associated First Person interview with the first author of the paper. Summary: Previous reports of axon initial segment Ranbp2 are shown to be based on NF186-cross-reacting antibodies and the intrinsic ability of an N-terminal fragment of Ranbp2 to bind to stable microtubules.