Location, Location, Location: Membrane Targeting Directed by PX Domains

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Abstract
Phosphoinositide (PI)–binding domains play critical roles in the intracellular localization of a variety of cell-signaling proteins. The 120–amino acid Phox homology (PX) domain targets proteins to organelle membranes through interactions between two conserved basic motifs within the PX domain and specific PIs. The combination of protein-lipid and protein-protein interactions ensures the proper localization and regulation of PX domain–containing proteins. Upon proper localization, PX domain–containing proteins can then bind to additional proteins and execute their functions in a diverse set of biological pathways, including intracellular protein transport, cell growth and survival, cytoskeletal organization, and neutrophil defense. With 30,000 to 40,000 genes potentially expressed in the human genome, cells face the difficult task of assembling these gene products into functional complexes and localizing them to appropriate sites. Of course, cells have developed a number of different strategies to deal with this problem, one of which is to spatially restrict proteins to their site of function and thus improve the probability that they will interact with their proper partners. In particular, the targeting of proteins to specific membrane-bound organelles has proven to be an effective cellular mechanism in maintaining the fidelity and efficiency of protein activities. Research within the past decade has identified protein domains that specifically bind the phosphatidylinositol (Ptd-Ins) phospholipids, collectively called phosphoinositides (PIs), as major determinants in localizing proteins to their site of function (1, 2). These PI-binding motifs, which include the C2 (PKC conserved region 2), PH (Pleckstrin homology), FYVE (Fab1p/YOTP/Vac1p/EEA1), ENTH (Epsin NH2-terminal homology) and tubby domains, are found in proteins implicated in a diverse array of cellular processes, such as protein transport, exocytosis, endocytosis, actin cytoskeletal organization, cell growth regulation, and control of gene expression. Through the regulated synthesis of distinct PIs on specific organelles, proteins containing these lipid-binding domains can be targeted and activated at the appropriate site of function. The importance of membrane targeting by PIs is exemplified by a number of human diseases linked to defects in PI signaling (3–5), including cancer, immunodeficiency disorders (X-linked agammaglobulinemina and chronic granulomatous disease), myotubular myopathy, kidney and neurological diseases (oculocerebro-renal syndrome of Lowe), and faciogenital dysplasia (Aarskog-Scott syndrome). Even with the large number of PI-binding proteins previously identified, genetic and biochemical studies suggest the existence of additional effector molecules. For example, it has long been known that PI synthesis is necessary for the generation of superoxides by the human NADPH oxidase complex, though the connection between these processes had been elusive. Recently, it was determined that Phox Homology (PX) domains, including those in two NADPH oxidase subunits, bind to PIs, identifying another family of effector proteins [(6–11); reviewed in (12)]. Many members of this effector family contain additional motifs that mediate protein-protein interactions and other biochemical activities, such as protein phosphorylation and lipid modification (13). As with other lipid-binding motifs, PX domains play important roles in ensuring that proteins reach their appropriate intracellular location through the binding of membrane-restricted PIs.