Developmentally Regulated Alternative Splicing in a Novel Synaptojanin*

Abstract

Phosphatidylinositol phosphates (PIPs) perform central functions in signal transduction and membrane traffic. Synaptojanin is a PIP 5-phosphatase that is expressed in a brain-specific and a ubiquitous splice variants and is thought to constitute the major PIP 5-phosphatase in mammalian brain (Woscholski, R., Finan, P. M., Radley, E., Totty, N. F., Sterling, A. E., Hsuan, J. J., Waterfield, M. D., and Parker, P. J. (1997) J. Biol. Chem. 272, 9625–9628). We now describe synaptojanin 2, a novel isoform of synaptojanin that, similar to synaptojanin 1, contains an N-terminal SAC1-like sequence and a central 5-phosphatase domain but a distinct, unique C-terminal sequence. Transfection studies demonstrated that synaptojanin 2, like synaptojanin 1, is an active PIP phosphatase. An interesting feature of synaptojanin 1 is the presence of a long open reading frame in the 3′ region of the brain mRNA that in non-brain tissues is joined to the coding region by alternative splicing, resulting in a shorter synaptojanin 1 form in brain and a longer form in peripheral tissues (Ramjaun, A. R., and McPherson, P. S. (1996) J. Biol. Chem. 271, 24856–24861). Although it exhibits no homology to synaptojanin 1 in this region, synaptojanin 2 also contains an open reading frame in the 3′ region that is subject to alternative splicing. Similar to synaptojanin 1, alternative splicing of synaptojanin 2 is tissue-specific and creates a shorter isoform expressed in brain and a longer form in peripheral tissues. The similar alternative splicing of two homologous proteins in a region of non-homology raises the possibility of evolutionary convergence and supports the significance of the variants. Analysis of mRNAs from three brain regions at different developmental stages revealed that alternative splicing of synaptojanin 2 is a developmentally late event, occurring only after the first postnatal week after the generation of neurons and initial synaptogenesis.