The Ubiquitin-Like Protein PLIC-1 or Ubiquilin 1 Inhibits TLR3-Trif Signaling

Background: The innate immune responses to virus infection are initiated by either Toll-like receptors (TLR3/7/8/9) or cytoplasmic double-stranded RNA (dsRNA)-recognizing RNA helicases RIG-I and MDA5. To avoid causing injury to the host, these signaling pathways must be switched off in time by negative regulators. Methodology/Principal Findings: Through yeast-two hybrid screening, we found that an ubiquitin-like protein named protein linking integrin-associated protein to cytoskeleton 1(PLIC-1 or Ubiquilin 1) interacted with the Toll/interleukin-1 receptor (TIR) domain of TLR4. Interestingly, PLIC-1 had modest effect on TLR4-mediated signaling, but strongly suppressed the transcriptional activation of IFN-β promoter through the TLR3-Trif-dependent pathway. Concomitantly, reduction of endogenous PLIC-1 by short-hairpin interfering RNA (shRNA) enhanced TLR3 activation both in luciferase reporter assays as well as in new castle disease virus (NDV) infected cells. An interaction between PLIC-1 and Trif was confirmed in co-immunoprecipitation (Co-IP) and GST-pull-down assays. Subsequent confocal microscopic analysis revealed that PLIC-1 and Trif colocalized with the autophagosome marker LC3 in punctate subcellular structures. Finally, overexpression of PLIC-1 decreased Trif protein abundance in a Nocodazole-sensitive manner. Conclusions: Our results suggest that PLIC-1 is a novel inhibitor of the TLR3-Trif antiviral pathway by reducing the abundance of Trif. © 2011 Biswas et al

The phospholipase complex PAFAH Ib regulates the functional organization of the Golgi complex

We report that platelet-activating factor acetylhydrolase (PAFAH) Ib, comprised of two phospholipase A(2) (PLA(2)) subunits, α1 and α2, and a third subunit, the dynein regulator lissencephaly 1 (LIS1), mediates the structure and function of the Golgi complex. Both α1 and α2 partially localize on Golgi membranes, and purified catalytically active, but not inactive α1 and α2 induce Golgi membrane tubule formation in a reconstitution system. Overexpression of wild-type or mutant α1 or α2 revealed that both PLA(2) activity and LIS1 are important for maintaining Golgi structure. Knockdown of PAFAH Ib subunits fragments the Golgi complex, inhibits tubule-mediated reassembly of intact Golgi ribbons, and slows secretion of cargo. Our results demonstrate a cooperative interplay between the PLA(2) activity of α1 and α2 with LIS1 to facilitate the functional organization of the Golgi complex, thereby suggesting a model that links phospholipid remodeling and membrane tubulation to dynein-dependent transport

A PLA1-2 punch regulates the Golgi complex

The mammalian Golgi complex, trans Golgi network (TGN) and ER-Golgi-Intermediate Compartment (ERGIC) are comprised of membrane cisternae, coated vesicles and membrane tubules, all of which contribute to membrane trafficking and maintenance of their unique architectures. Recently, a new cast of players was discovered to regulate the Golgi and ERGIC: four unrelated cytoplasmic phospholipase A (PLA) enzymes, cPLA(2)α (GIVA cPLA(2)), PAFAH Ib (GVIII PLA(2)), iPLA(2)-β (GVIA-2 iPLA(2)), and iPLA(1)γ. These ubiquitously expressed enzymes regulate membrane trafficking from specific Golgi subcompartments, although there is evidence for some functional redundancy between PAFAH Ib and cPLA(2)α. Three of these enzymes, PAFAH Ib, cPLA(2)α, and iPLA(2)-β, exert effects on Golgi structure and function by inducing the formation of membrane tubules. Here, we review our current understanding of how PLA enzymes regulate Golgi and ERGIC morphology and function

Eps15: a multifunctional adaptor protein regulating intracellular trafficking

<p>Abstract</p> <p>Over expression of receptor tyrosine kinases is responsible for the development of a wide variety of malignancies. Termination of growth factor signaling is primarily determined by the down regulation of active growth factor/receptor complexes. In recent years, considerable insight has been gained in the endocytosis and degradation of growth factor receptors. A crucial player in this process is the EGFR Protein tyrosine kinase Substrate #15, or Eps15. This protein functions as a scaffolding adaptor protein and is involved both in secretion and endocytosis. Eps15 has been shown to bind to AP-1 and AP-2 complexes, to bind to inositol lipids and to several other proteins involved in the regulation of intracellular trafficking. In addition, Eps15 has been detected in the nucleus of mammalian cells. Activation of growth factor receptors induces tyrosine phosphorylation and mono-ubiquitination of Eps15. The role of these post translational modifications of Eps15 is still a mystery. It is proposed that Eps15 and its family members Eps15R and Eps15b are involved in the regulation of membrane morphology, which is required for intracellular vesicle formation and trafficking.</p