Molecular characterization of MARTA1, a protein interacting with the dendritic targeting element of MAP2 mRNAs

In neurones, the somatodendritic microtubule-associated protein 2 regulates the stability of the dendritic cytoskeleton. Its extrasomatic localization appears to be a multicausal mechanism that involves dendritic mRNA trafficking, a process that depends on a dendritic targeting element in the 3' untranslated region. Two rat MAP2-RNA trans-acting proteins, MARTA1 and MARTA2, exhibit specific high-affinity binding to the dendritic targeting element. We have now affinity-purified MARTA1 from rat brain. Analysis of proteolytic peptides revealed that rat MARTA1 is the orthologue of the human RNA-binding protein KSRP. Rat MARTA1 is a 74-kDa protein that contains four putative RNA-binding domains and is 98% identical to human KSRP. Both purified rat MARTA1 and human KSRP preferentially bind to the dendritic targeting element, but do not strongly interact with other investigated regions of mRNAs encoding microtubule-associated protein 2 and alpha-tubulin. In rat brain neurones and cultured neurones derived from superior cervical ganglia, MARTA1 is primarily intranuclear, but is also present in the somatodendritic cytoplasm. Thus, MARTA1 may play a role in nucleocytoplasmic mRNA targeting.