SYNCRIP, a cytoplasmic counterpart of heterogeneous nuclear ribonucleoprotein R, interacts with ubiquitous synaptotagmin isoforms

Synaptotagmins (Syts) are a large family of membrane proteins consisted of at least 12 isoforms. They are categorized in neuron-specific isoforms (I- V, X, and XI) and ubiquitous isoforms (VI-IX) based on their expression patterns. Syt-I, a neuron-specific and abundant isoform, has been well characterized and postulated to be the exocytotic Ca²⁺ sensor. However, the functions of other isoforms remain obscure. Here, we report that ubiquitous isoforms of synaptotagmins, Syt-VII, Syt-VIII, and Syt-IX, interacted with a cytoplasmic RNA-binding protein, SYNCRIP (Synaptotagmin-binding, cytoplasmic RNA-interacting protein), through their C2B domains. SYNCRIP was originally found in the Syt-II C2AB domain bound fraction from the mouse brain lysate, cDNA cloning of SYNCRIP cDNA revealed that the protein was highly homologous to heterogeneous nuclear ribonucleoprotein R (hnRNP R) recently identified. SYNCRIP protein was ubiquitously and constantly expressed in various tissues of mice parallel to hnRNP R. SYNCRIP indeed bound RNA with preference to poly(A) RNA; however, in contrast to the nuclear localization of hnRNP R, SYNCRIP was distributed predominantly in the cytoplasm as judged by both biochemical fractionation and immunohistochemical studies. In vitro binding experiments showed the potential interaction of SYNCRIP with C2B domains of Syts except for those of Syt-V,-VI, and -X. Furthermore, the interaction between SYNCRIP and Syt-VII, -VIII, or -IX was revealed by coimmunoprecipitation experiments using COS cells transiently expressing each Syt isoform. These findings suggested that SYNCRIP was a target of ubiquitous type of Syts and implied the involvement of ubiquitous Syts in the regulation of dynamics of the cytoplasmic mRNA.