The specificity of protein-protein interactions in cellular signaling cascades is dependent on the sequence and intramolecular location of distinct amino acid motifs. We used the two-hybrid ...interaction trap to identify proteins that can associate with the PDZ motif-rich segment in the protein tyrosine phosphatase PTP-BL. A specific interaction was found with the Lin-11, Isl-1, Mec-3 (LIM) domain containing protein RIL. More detailed analysis demonstrated that the binding specificity resides in the second and fourth PDZ motif of PTP-BL and the LIM domain in RIL. Immunohistochemistry on various mouse tissues revealed a submembranous colocalization of PTP-BL and RIL in epithelial cells. Remarkably, there is also an N-terminal PDZ motif in RIL itself that can bind to the RIL-LIM domain. We demonstrate here that the RIL-LIM domain can be phosphorylated on tyrosine in vitro and in vivo and can be dephosphorylated in vitro by the PTPase domain of PTP-BL. Our data point to the presence of a double PDZ-binding interface on the RIL-LIM domain and suggest tyrosine phosphorylation as a regulatory mechanism for LIM-PDZ associations in the assembly of multiprotein complexes. These findings are in line with an important role of PDZ-mediated interactions in the shaping and organization of submembranous microenvironments of polarized cells.
Expansion of a Glutamine (Gln) repeat above a specific critical size in certain proteins gives rise to aggregation-prone proteins that cause neurodegenerative disorders, such as Huntington's disease. ...However, proteins with long hydrophilic polyglutamine repeats are more frequently found in nature than proteins with long homogeneous repeats of other amino acids, such as hydrophobic (Ala)n and (Leu)n. To explore this finding, the effects of expression in mammalian cells of polyglutamine and polyleucine encoded by mixed DNA repeats were compared. It was found that polyleucine is significantly more toxic than polyglutamine. In addition, we show that polyleucine stretches display a high propensity for aggregation utilizing two complementary biochemical assays and that polyleucine stretches can also be detected by the monoclonal antibody 1C2, which specifically recognizes expanded pathogenic and aggregation-prone glutamine repeats. Together, these results suggest that polyglutamine stretches are in fact relatively well tolerated and that nature may select more strongly against DNA stretches that encode long hydrophobic homopolymeric amino acid stretches, such as polyleucine – possibly owing to their strong propensity for aggregation. In keeping with this notion, an increasing number of diseases are found to be associated with expansion of stretches of hydrophobic amino acids, including oculopharyngeal muscular dystrophy (OPMD), which is associated with expansion of a hydrophobic polyalanine stretch.
We here describe the identification and characterization of a novel bromodomain-containing protein, the bromodomain protein of 75 kDa (BP75). Initially, we identified BP75 in a two-hybrid screening ...for proteins that interact with the first PDZ (acronym for post-synaptic density protein PSD-95,
Drosophila discs large tumor suppressor DlgA and the tight junction protein ZO-1) domain in protein tyrosine phosphatase-BAS-like (PTP-BL). We found that BP75 is expressed ubiquitously and show that both BP75 and a PTP-BL deletion mutant consisting of the first PDZ domain are located mainly in the nucleus, although cytoplasmic localization is also evident. Full-length PTP-BL, on the contrary, is predominantly localized in the cytoplasm, although some basal nuclear staining is observed. The described molecular interaction may reflect a mechanism of coupling submembraneous signalling events and nuclear events.
The mouse gene Ptprr encodes the neuronal protein tyrosine phosphatases PTP-SL and PTPBR7. These proteins differ in their N-terminal domains, with PTP-SL being a cytosolic, membrane-associated ...phosphatase and PTPBR7 a type I transmembrane protein. In this study, we further explored the nature of the PTP-SL-associated vesicles in neuronal cells using a panel of organelle markers and noted a comparable subcellular distribution for PTP-SL and the beta4-adaptin subunit of the AP4 complex. PTP-SL, PTPBR7 and beta4-adaptin are localised at the Golgi apparatus and at vesicles throughout the cytoplasm. Immunohistochemical analysis demonstrated that PTP-SL, PTPBR7 and beta4-adaptin are all endogenously expressed in brain. Interestingly, coexpression of PTP-SL and beta4-adaptin leads to an altered subcellular localisation for PTP-SL. Instead of the Golgi and vesicle-type staining pattern, still observable for beta4-adaptin, PTP-SL is now distributed throughout the cytoplasm. Although beta4-adaptin was found to interact with the phosphatase domain of PTP-SL and PTPBR7 in the yeast two-hybrid system, it failed to do so in transfected neuronal cells. Our data suggest that the tyrosine phosphatases PTP-SL and PTPBR7 may be involved in the formation and transport of AP4-coated vesicles or in the dephosphorylation of their transmembrane cargo molecules at or near the Golgi apparatus.
The mouse gene Ptprr encodes the neuronal protein tyrosine phosphatases PTP-SL and PTPBR7. These proteins differ in their N-terminal domains, with PTP-SL being a cytosolic, membrane-associated ...phosphatase and PTPBR7 a type I transmembrane protein. In this study, we further explored the nature of the PTP-SL-associated vesicles in neuronal cells using a panel of organelle markers and noted a comparable subcellular distribution for PTP-SL and the β4-adaptin subunit of the AP4 complex. PTP-SL, PTPBR7 and β4-adaptin are localised at the Golgi apparatus and at vesicles throughout the cytoplasm. Immunohistochemical analysis demonstrated that PTP-SL, PTPBR7 and β4-adaptin are all endogenously expressed in brain. Interestingly, coexpression of PTP-SL and β4-adaptin leads to an altered subcellular localisation for PTP-SL. Instead of the Golgi and vesicle-type staining pattern, still observable for β4-adaptin, PTP-SL is now distributed throughout the cytoplasm. Although β4-adaptin was found to interact with the phosphatase domain of PTP-SL and PTPBR7 in the yeast two-hybrid system, it failed to do so in transfected neuronal cells. Our data suggest that the tyrosine phosphatases PTP-SL and PTPBR7 may be involved in the formation and transport of AP4-coated vesicles or in the dephosphorylation of their transmembrane cargo molecules at or near the Golgi apparatus.
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