IDENTIFICATION OF A FAMILY OF SORTING NEXIN MOLECULES AND CHARACTERIZATION OF THEIR ASSOCIATION WITH RECEPTORS

Sorting nexin 1 (SNX1) is a protein that binds to the epidermal growth factor (EGF) receptor and is proposed to play a role in directing EGF receptors to lysosomes for degradation (R.C. Kurten, D. L. Cadena, an d G. N. Gill, Science 272:1008-1010, 1996), We have obtained full-leng th cDNAs and deduced the amino acid sequences of three novel homologou s proteins, which were denoted human sorting nexins (SNX2, SNX3, and S NX4), In addition, we identified a presumed splice variant isoform of SNX1 (SNX1A). These molecules contain a conserved domain of similar to 100 amino acids, which was termed the phox homology (PX) domain. Huma n SNX1 (522 amino acids), SNX1A (457 amino acids), SNX2 (519 amino aci ds), SNX3 (162 amino acids), and SNX4 (450 amino acids) are part of a larger family of hydrophilic molecules including proteins identified i n Caenorhabditis elegans and Saccharomyces cerevisiae, Despite their h ydrophilic nature, the sorting nexins are found partially associated w ith cellular membranes. They are widely expressed, although the tissue distribution of each sorting nexin mRNA varies. When expressed in COS 7 cells, epitope-tagged sorting nexins SNX1, SNX1A, SNX2, and SNX4 coi mmunoprecipitated with receptor tyrosine kinases for EGF, platelet-der ived growth factor, and insulin. These sorting nexins also associated with the long isoform of the leptin receptor but not with the short an d medium isoforms. Interestingly, endogenous COS7 transferrin receptor s associated exclusively with SNX1 and SNX1A, while SNX3 was not found to associate with any of the receptors studied. Our demonstration of a large conserved family of sorting nexins that interact with a variet y of receptor types suggests that these proteins may be involved in se veral stages of intracellular trafficking in mammalian cells.