High-tailing it to the apical surface. Focus on "Apical targeting of the P2Y(4) receptor is directed by hydrophobic and basic residues in the cytoplasmic tail".

EPITHELIAL CELL FUNCTION REQUIRES the generation and maintenance of polarized apical and basolateral membrane domains with distinct protein and lipid compositions. This asymmetry in membrane composition is essential for myriad cell functions that include substratum adhesion, cell-cell communication, nutrient uptake, ion transport, and signal transduction. Genetic mutations that interfere with the polarized targeting or routing of plasma membrane proteins can result in severe disease states (6). Identifying the signals and machinery that direct the initial and postendocytic delivery of proteins to either surface domain has been the subject of intense investigation over the past three decades. Nonetheless, many questions remain about how diverse classes of proteins with distinct membrane topologies are targeted, often with exquisite fidelity, to the apical or basolateral surface. Apical sorting signals are diverse and can be found on the luminal, membrane-associated, or cytoplasmically disposed aspects of various proteins (10). For some proteins, the apical sorting determinants involve posttranslational modifications, including a glycosyl-phosphatidylinositol anchor or Nor Olinked glycans. In addition, there is a rapidly growing list of proteins sorted to the apical surface via peptide sequences within cytoplasmically oriented loops or tails of proteins. In most cases, these peptide-based motifs have been identified on polytopic proteins that span the membrane multiple times; however, apical sorting information on the single membranespanning receptor megalin has also been localized to its cytoplasmic tail (7, 8). To date, no similarity among these signals, either in length or in sequence, has been noted, and how these motifs are interpreted by cellular machinery to support precise cargo routing remains a mystery. One important class of polytopic proteins that mediates cellular responses to external signals is the nucleotide P2Y receptors. These members of the G protein-coupled receptor (GPCR) family bind to extracellular ATP and nucleotides and regulate cellular processes via both paracrine and autocrine mechanisms (2). The P2Y receptor family includes eight nonsequentially named receptors that are variously coupled to heterotrimeric G proteins, such as to Gq or Gi that mobilize intracellular Ca or inhibit adenylyl cyclase, respectively (9). The P2Y4 receptor, which is the focus of the article by DuBose et al. (3) in this issue of American Journal of Physiology-Cell Physiology, is expressed in numerous tissues. In intestinal cells, activation of the receptor induces luminal Cl secretion, and the development of targeted P2Y4 agonists and antagonists has been proposed as promising potential strategies for the treatment of diseases as diverse as cystic fibrosis and infectious diarrhea. Individual P2Y receptors are typically distributed to either the apical or the basolateral surfaces of polarized lung, kidney, and intestinal epithelial cells (12). At steady state, P2Y2, P2Y4, and P2Y6 receptors are expressed apically, whereas P2Y1, P2Y11, P2Y12, and P2Y14 receptors are enriched at the basolateral membrane, and P2Y13 is present at both domains. The differential distribution of these related family members thus provides a unique opportunity to dissect polarized sorting motifs on polytopic proteins using a chimera approach. Indeed, previous studies have identified the apical sorting determinant of the P2Y2 receptor using this strategy (5). The apical sorting signal of P2Y2 is striking in that it requires two stretches of amino acids localized within a luminally oriented loop of the protein, and may thus represent a new class of peptide-based apical targeting signal (5). The article by DuBose et al. describes the systematic dissection of apical targeting information in the cytoplasmic tail of the P2Y4 receptor (3). Previous studies by this group demonstrated that deletion of the 55 amino acid tail of P2Y4