Identification and characterization of a novel Delphilin variant with an alternative N-terminus

Delphilin is identified as a Glutamate receptor δ2 (GluRδ2) subunit interacting protein, consisting of a PDZ domain and formin homology (FH) domains 1 and 2, in addition to a C-terminal coiled-coil structure. Delphilin has been shown to be selectively expressed in cerebellar Purkinje cells where it co-localizes with the GluRδ2 subunit at the Purkinje cell-parallel fiber synapses. Although Delphilin specifically interacts with the GluRδ2 C-terminus via its PDZ domain, the physiological role of the interaction is not yet understood. Here, we report that the Delphilin protein exhibits diversity at its N-terminus by variable usage of the first several exons. Interestingly, the two Delphilin mRNAs which correspond to the first one initially identified (now designated as Delphilin α) and the second that contains a newly identified first exon (designated as Delphilin β), show different chronological expression profiles. Delphilin β mRNA was not decreased throughout the cerebellar development in vivo and in vitro, while in vivo Delphilin α mRNA gradually decreases following the first postnatal week. Delphilins α and β also revealed different subcellular distribution with some overlap. Specifically, the cerebellar synaptosomal membrane fraction contained the Delphilin β protein. Both Delphilin α and β localized at the dendritic spines with GluRδ2; however, dendritic shafts in cultured Purkinje cells also included Delphilin β. In MDCK cells upon becoming confluent, Delphilin α moved to the cell-cell junction area, whereas Delphilin β maintained a diffuse distribution pattern throughout the cytoplasm. Taken as a whole, these two different Delphilins seemed to play functionally different roles in developing and matured cerebellar Purkinje cells.