Identification of a Family of Sorting Nexin Molecules and Characterization of Their Association with Receptors

Abstract

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, and G. N. Gill, Science 272:1008-1010, 1996). We have obtained full-length cDNAs and deduced the amino acid sequences of three novel homologous proteins, which were denoted human sorting nexins (SNX2, SNX3, and SNX4). In addition, we identified a presumed splice variant isoform of SNX1 (SNX1A). These molecules contain a conserved domain of ϳ100 amino acids, which was termed the phox homology (PX) domain. Human SNX1 (522 amino acids), SNX1A (457 amino acids), SNX2 (519 amino acids), SNX3 (162 amino acids), and SNX4 (450 amino acids) are part of a larger family of hydrophilic molecules including proteins identified in Caenorhabditis elegans and Saccharomyces cerevisiae. Despite their hydrophilic nature, the sorting nexins are found partially associated with cellular membranes. They are widely expressed, although the tissue distribution of each sorting nexin mRNA varies. When expressed in COS7 cells, epitope-tagged sorting nexins SNX1, SNX1A, SNX2, and SNX4 coimmunoprecipitated with receptor tyrosine kinases for EGF, platelet-derived growth factor, and insulin. These sorting nexins also associated with the long isoform of the leptin receptor but not with the short and medium isoforms. Interestingly, endogenous COS7 transferrin receptors 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 variety of receptor types suggests that these proteins may be involved in several stages of intracellular trafficking in mammalian cells.

A Retromerlike Complex Is a Novel Rab7 Effector That Is Involved in the Transport of the Virulence Factor Cysteine Protease in the Enteric Protozoan ParasiteEntamoeba histolytica

“…It was shown by siRNA that knockdown of SNX1 showed no effect on EGF receptor degradation (Gullapalli et al, 2004;Carlton et al, 2004) or on transferrin receptor trafficking (Carlton et al, 2004), whereas SNX1 is required for proper endosome retrieval of the CI-MPR (Carlton et al, 2005). This contrasts with the findings of previous studies using overexpression showing that overexpression of SNX1 enhances EGF receptor degradation (Kurten et al, 1996;Chin et al, 2001;Zheng et al, 2001), whereas other groups were unable to demonstrate that overexpression of SNX1 or SNX2, 3, or 4 increases the turnover of insulin or EGF receptors (Haft et al, 1998).…”

Regulation of cell proliferation by Smad proteins

“…Overexpression of Cav-1 was reported to reduce TGF-binduced Smad activation (Razani et al, 2001). Furthermore, the family of sorting nexins (SNXs) that are intracellular traf®cking molecules of receptor tyrosine kinases (Haft et al, 1998), also interact with TGF-b receptors (Parks et al, 2001).…”

Phosphoinositides in Constitutive Membrane Traffic

“…PX, or phox (phagocyte oxidase), homology domains are found in two subunits of NADPH-oxidase, in PI3KC2␥ and in a family of small proteins called sorting nexins (SNX) (128,277). They are also found in certain other yeast proteins known to be required for protein traffic between the Golgi and endosomes (91,153,368) and in phospholipase D 1 (PLD 1 ) and phospholipase D 2 (PLD 2 ) (105) ( Table 4).…”

“…The first sorting nexin family member, SNX1, had been identified as a protein that bound to the EGF receptor and that had sequence homology to a yeast protein known to function in membrane traffic to the vacuole (192). Subsequently, a number of SNX proteins have been shown to associate with specific membrane receptor proteins as part of an oligomeric complex that regulates sorting of receptors between recycling and degradation pathways (128,153,342). SNX proteins appear to associate into complexes with other SNX proteins, as well as with adaptor proteins that bind to receptors and to clathrin (193,206,211,267).…”