Megalin and the low-density lipoprotein (LDL) receptorrelated protein (LRP) are two large members of the LDL receptor family that bind and endocytose multiple ligands. The molecular and cellular determinants that dictate the sorting behavior of these receptors in polarized epithelial cells are largely unknown. Megalin is found apically distributed, whereas the limited information on LRP indicates its polarity. We show here that in Madin-Darby canine kidney cells, both endogenous LRP and a minireceptor containing the fourth ligand-binding, transmembrane and LRP cytosolic domains were basolaterally sorted. In contrast, minireceptors that either lacked the cytoplasmic domain or had the tyrosine in the NPTY motif mutated to alanine showed a preferential apical distribution. In LLC-PK1 cells, endogenous megalin was found exclusively in the apical membrane. Studies were also done using chimeric proteins harboring the cytosolic tail of megalin, one with the fourth ligand-binding domain of LRP and the other two containing the green fluorescent protein as the ectodomain and transmembrane domains of either megalin or LRP. Findings from these experiments showed that the cytosolic domain of megalin is sufficient for apical sorting, and that the megalin transmembrane domain promotes association with lipid rafts.In conclusion, we show that LRP and megalin both contain sorting information in their cytosolic domains that directs opposite polarity, basolateral for LRP and apical for megalin. Additionally, we show that the NPTY motif in LRP is important for basolateral sorting and the megalin transmembrane domain directs association with lipid rafts. The low-density lipoprotein (LDL) receptor gene family contains three very large members, LRP (LRP1), a dimer of 515 kDa and 85 kDa, its closely related homolog, megalin (LRP2), a single species of 600 kDa (1) and the recently discovered LRP1B, more closely related to LRP than to megalin (2). Comparison of LRP and megalin reveals that the overall protein domain structural organization of the two proteins is very similar. LRP has four ligandbinding domains (I, II, III, and IV from the N-terminus) separated from one another by clusters of EGF-precursor repeats and F/YWXD spacer repeats. LRP contains a furin endopeptidase processing site in its ectodomain that is cleaved to form the mature receptor, a noncovalently associated heterodimer, consisting of an extracellular 515-kDa subunit and a transmembrane 85-kDa subunit (3, 4). The cytoplasmic tail of LRP has 100 amino acids and harbors two NPxY motifs, one Yxxf motif, recently shown as the dominant endocytosis motif (5), and two LL motifs, with the distal one playing a small role in LRP internalization. In addition to these motifs, phosphorylation of the LRP tail by PKA also plays a role in the internalization of the receptor (6). Megalin contains four clusters of ligand-binding domains. The first ligand-binding domain is the most different from the corresponding domain in LRP, with seven ligand binding repeats instead of two (7). The t...

In non-neuronal cells, inactivation of protein kinase D (PKD) blocks fission of trans-Golgi network (TGN) transport carriers, inducing the appearance of long tubules filled with cargo. We now report on the function of PKD1 in neuronal protein trafficking. In cultured hippocampal pyramidal cells, the transferrin receptor (TfR) and the low-density receptor-related protein (LRP) are predominantly transported to dendrites and excluded from axons. Expression of kinase-inactive PKD1 or its depletion by RNA interference treatment dramatically and selectively alter the intracellular trafficking and membrane delivery of TfR-and LRP-containing vesicles, without inhibiting exit from the TGN or inducing Golgi tubulation. After PKD1 suppression, dendritic membrane proteins are mispackaged into carriers that transport VAMP2; these vesicles are distributed to both axons and dendrites, but are rapidly endocytosed from dendrites and preferentially delivered to the axonal membrane. A kinase-defective mutant of PKD1 lacking the ability to bind diacylglycerol and hence its Golgi localization does not cause missorting of TfR or LRP. These results suggest that in neurons PKD1 regulates TGN-derived sorting of dendritic proteins and hence has a role in neuronal polarity.

In addition to hemostasis, human platelets have several immune functions and interact with infectious pathogens including HIV in vitro. Here, we report that platelets from HIV-infected individuals on combined antiretroviral drug therapy (ART) with low blood CD4+ T cell counts (<350 cells/μl) contained replication-competent HIV despite viral suppression. In vitro, human platelets harboring HIV propagated the virus to macrophages, a process that could be prevented with the biologic abciximab, an anti–integrin αIIb/β3 Fab. Furthermore, in our cohort, 88% of HIV-infected individuals on ART with viral suppression and with platelets containing HIV were poor immunological responders with CD4+ T cell counts remaining below <350 cells/μl for more than one year. Our study suggests that platelets may be transient carriers of HIV and may provide an alternative pathway for HIV dissemination in HIV-infected individuals on ART with viral suppression and poor CD4+ T cell recovery.

Low-density lipoprotein receptor-related protein 1 (LRP1) is an endocytic recycling receptor with two cytoplasmic tyrosine-based basolateral sorting signals. Here we show that during biosynthetic trafficking LRP1 uses AP1B adaptor complex to move from a post-TGN recycling endosome (RE) to the basolateral membrane. Then it recycles basolaterally from the basolateral sorting endosome (BSE) involving recognition by sorting nexin 17 (SNX17).In the biosynthetic pathway, Y 29 but not N 26 from a proximal NPXY directs LRP1 basolateral sorting from the TGN. A N 26 A mutant revealed that this NPXY motif recognized by SNX17 is required for the receptor's exit from BSE. An endocytic Y 63 ATL 66 motif also functions in basolateral recycling, in concert with an additional endocytic motif (LL 86,87 ), by preventing LRP1 entry into the transcytotic apical pathway. All this sorting information operates similarly in hippocampal neurons to mediate LRP1 somatodendritic distribution regardless of the absence of AP1B in neurons. LRP1 basolateral distribution results then from spatially and temporally segregation steps mediated by recognition of distinct tyrosine-based motifs. We also demonstrate a novel function of SNX17 in basolateral/somatodendritic recycling from a different compartment than AP1B endosomes. INTRODUCTIONEpithelial cells posses functional, morphological, and biochemically distinct apical and basolateral cell surface domains and maintain this polarized phenotype addressing specific plasma membrane proteins into each domain (Yeaman et al., 1999;Mostov, 2003;Rodriguez-Boulan et al., 2005). Apical and basolateral proteins are sorted in the biosynthetic route at the level of the trans-Golgi network (TGN; Rindler et al., 1984;Fuller et al., 1985;Griffiths and Simons, 1986), and those proteins that undergo endocytosis can be additionally sorted in recycling endosomes (RE;Mostov and Cardone, 1995;Odorizzi and Trowbridge, 1997). Evidence accumulated over a decade and consolidated in the most recent studies (Ang et al., 2004;Lock and Stow, 2005;Cancino et al., 2007;Cresawn et al., 2007;Gravotta et al., 2007) have shown that the biosynthetic route of at least some proteins includes a post-TGN transit through RE. Under this scenery, it is now important to define the relative contribution of the TGN and RE in the polarized sorting mechanisms of different cargo and in different kind of polarized cells. Neurons, for instance, have to direct distinct proteins to somato-dendritic or axonal plasma membrane domains (Rodriguez-Boulan and Powell, 1992;Winckler and Mellman, 1999), yet their protein-sorting mechanisms remain less known than in epithelial cells. A comparative analysis in epithelial cells and neurons could indeed help to understand the underlying mechanisms of the polarized phenotype.Studies in MDCK cells, the most currently used model of cell polarity, settled the basics of apical and basolateral protein sorting (Rodriguez-Boulan et al., 2005). Apical membrane proteins possess sorting information located in their extracel...

SUMMARYUridine 5¢-diphosphate (UDP)-glucose is transported into the lumen of the endoplasmic reticulum (ER), and the Arabidopsis nucleotide sugar transporter AtUTr1 has been proposed to play a role in this process; however, different lines of evidence suggest that another transporter(s) may also be involved. Here we show that AtUTr3 is involved in the transport of UDP-glucose and is located at the ER but also at the Golgi. Insertional mutants in AtUTr3 showed no obvious phenotype. Biochemical analysis in both AtUTr1 and AtUTr3 mutants indicates that uptake of UDP-glucose into the ER is mostly driven by these two transporters. Interestingly, the expression of AtUTr3 is induced by stimuli that trigger the unfolded protein response (UPR), a phenomenon also observed for AtUTr1, suggesting that both AtUTr1 and AtUTr3 are involved in supplying UDP-glucose into the ER lumen when misfolded proteins are accumulated. Disruption of both AtUTr1 and AtUTr3 causes lethality. Genetic analysis showed that the atutr1 atutr3 combination was not transmitted by pollen and was poorly transmitted by the ovules. Cell biology analysis indicates that knocking out both genes leads to abnormalities in both male and female germ line development. These results show that the nucleotide sugar transporters AtUTr1 and AtUTr3 are required for the incorporation of UDP-glucose into the ER, are essential for pollen development and are needed for embryo sac progress in Arabidopsis thaliana.