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Volume 272, Number 49, Issue of December 5, 1997 pp. 30817-30821
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

Synaptojanin 2, a Novel Synaptojanin Isoform with a Distinct Targeting Domain and Expression Pattern

(Received for publication, August 15, 1997)

From the Department of Cell Biology and Howard Hughes Medical Institute, Yale University, School of Medicine, New Haven, Connecticut 06510

Synaptojanin (synaptojanin 1) is a recently identified inositol 5-phosphatase, which is highly enriched in nerve terminals and is implicated in synaptic vesicle recycling. It is composed of three domains: an amino-terminal SacI homology region, a central inositol 5-phosphatase homology region, and a carboxyl-terminal proline-rich region. We have now identified and characterized a novel form of synaptojanin, synaptojanin 2, which has a broader tissue distribution. Synaptojanin 2 cDNA from rat brain library encodes a protein of 1,248 amino acids with a predicted M_r of 138,268. The two synaptojanin isoforms share 57.2 and 53.8% amino acid identity in their SacI and phosphatase domains, respectively. In marked contrast, their carboxyl-terminal proline-rich regions bear little homology. Expression of synaptojanin 2 in COS7 cells produced a 140-kDa protein with inositol 5-phosphatase actvity. Protein binding assays demonstrated that among the major src homology 3-proteins known to bind to the proline-rich region of synaptojanin 1, Grb2, amphiphysin, and members of SH3p4/8/13 protein family, only Grb2 bound to that of synaptojanin 2. Furthermore, subcellular fractionation studies in transfected Chinese hamster ovary cells revealed that synaptojanin 2 was predominantly associated with the particulate fraction while synaptojanin 1 was mainly localized in the soluble fraction. This observation suggests that the proline-rich regions of synaptojanins 1 and 2 are implicated in different protein-protein interactions and direct the two isoforms to different subcellular compartments.

Our results demonstrate the presence of a family of synaptojanin-type inositol 5-phosphatases with different tissue and subcellular distributions, which may be involved in distinct membrane trafficking and signal transduction pathways in mammalian cells.

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