Author Posting. © The Author(s), 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Neuroscience 166 (2010): 73-83, doi:10.1016/j.neuroscience.2009.12.005. ; A polyclonal antibody (C4), raised against the head domain of chicken myosin Va, reacted strongly towards a 65 kDa polypeptide (p65) on western blots of extracts from squid optic lobes but did not recognize the heavy chain of squid myosin V. This peptide was not recognized by other myosin Va antibodies, nor by an antibody specific for squid myosin V. In an attempt to identify it, p65 was purified from optic lobes of Loligo plei by cationic exchange and reverse phase chromatography. Several peptide sequences were obtained by mass spectroscopy from p65 cut from SDS-PAGE gels. BLAST analysis and partial matching with ESTs from a Loligo pealei data bank indicated that p65 contains consensus signatures for the hnRNP A/B family of RNA-binding proteins. Centrifugation of post mitochondrial extracts from optic lobes on sucrose gradients after treatment with RNase gave biochemical evidence that p65 associates with cytoplasmic RNP complexes in an RNA-dependent manner. Immunohistochemistry and immunofluorescence studies using the C4 antibody showed partial co-labeling with an antibody against squid synaptotagmin in bands within the outer plexiform layer of the optic lobes and at the presynaptic zone of the stellate ganglion. Also, punctate labeling by the C4 antibody was observed within isolated optic lobe synaptosomes. The data indicate that p65 is a novel RNA-binding protein located to the presynaptic terminal within squid neurons and may have a role in synaptic localization of RNA and its translation or processing. ; REL, JCR and JEM received financial support from the Fundação de Amparo à Pesquisa do Estado de Sao Paulo (FAPESP), the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and the Fundação de Apoio ao Ensino, Pesquisa e Assistência do Hospital das Clínicas da FMRP-USP (FAEPA). JAD received financial support from the RI-INBRE Program Grant #P20RR016457 from the Nation Center for Research Resources, NIH, Bethesda, MD. DTPL, LC, SBFT, EJRV and MMAB were recipients of research fellowships from FAPESP and CNPq. REL and JEM received Productivityin- Research fellowships from CNPq.