The sorting nexin, DSH3PX1, connects the axonal guidance receptor, dscam, to the actin cytoskeleton

Dock, an adaptor protein that functions in Drosophila axonal guidance, cons ists of three tandem Src homology 3 (SH3) domains preceding an SH2 domain. To develop a better understanding of axonal guidance at the molecular level , we used the SH2 domain of Dock to purify a protein complex from fly S2 ce lls. Five proteins were obtained in pure form from this protein complex. Th e largest protein in the complex was identified as Dscam ((D) under bar own (s) under bar yndrome (c) under bar ell (a) under bar dhesion (m) under ba r olecule), which was subsequently shown to play a key role in directing ne urons of the fly embryo to correct positions within the nervous system (Sch mucker, D., Clemens, J. C., Shu, H., Worby, C. A., Xiao, J., Muda, M., Dixo n, J. E., and Zipursky, S. L. (2000) Cell 101, 671-684). The smallest prote in in this complex (p63) has now been identified. We have named p63 DSH3PXI because it appears to be the Drosophila orthologue of the human protein kn own as SH3PX1. DSH3PX1 is comprised of an NH2-terminal SH3 domain, an inter nal PHOX homology (PX) domain, and a carboxyl-terminal coiled-coil region. Because of its PX domain, DSH3PX1 is considered to be a member of a growing family of proteins known collectively as sorting nexins, some of which hav e been shown to be involved in vesicular trafficking. We demonstrate that D SH3PX1 immunoprecipitates with Dock and Dscam from S2 cell extracts. The do mains responsible for the in vitro interaction between DSH3PX1 and Dock wer e also identified. We further show that DSH3PX1 interacts with the Drosophi la orthologue of Wasp, a protein component of actin polymerization machiner y, and that DSH3PX1 coimmunoprecipitates with AP-50, the clathrin-coat adap ter protein. This evidence places DSH3PX1 in a complex linking cell surface receptors like Dscam to proteins involved in cytoskeletal rearrangements a nd/or receptor trafficking.