The ubiquitin-proteasome pathway (UPP) and autophagy play integral roles in cellular homeostasis. As part of their normal life cycle, most proteins undergo ubiquitination for some form of redistribution, localization and/or functional modulation. However, ubiquitination is also important to the UPP and several autophagic processes. The UPP is initiated after specific lysine residues of short-lived, damaged or misfolded proteins are conjugated to ubiquitin, which targets these proteins to proteasomes. Autophagy is the endosomal/lysosomal-dependent degradation of organelles, invading microbes, zymogen granules and macromolecules such as protein, carbohydrates and lipids. Autophagy can be broadly separated into three distinct subtypes termed microautophagy, chaperone-mediated autophagy and macroautophagy. Although autophagy was once thought of as non-selective bulk degradation, advancements in the field have led to the discovery of several selective forms of autophagy. Here, we focus on the mechanisms of primary and selective mammalian autophagy pathways and highlight the current knowledge gaps in these molecular pathways.

The TGFβ (transforming growth factor β) pathway is an essential cell signalling pathway that is implicated in both normal developmental processes, such as organogenesis, and pathological disorders, such as cancer and fibrosis. There are three prototypical TβRs (TGFβ receptors): TβRI (TβR type I), TGβRII (TβR type II) and TGFβRIII (TβR type III, also known as betaglycan). Whereas the role of TβRII and TβRI in TGFβ signal propagation has been established, the contribution of TβRIII to TGFβ signalling is less well understood. At the cell surface, TβRI and TβRII receptors can be internalized by clathrin-mediated endocytosis and clathrin-independent membrane-raft-dependent endocytosis. Interestingly, the endocytic route of the receptors plays a direct role in TGFβ-dependent Smad signal transduction; receptors endocytosed via clathrin-mediated endocytosis activate Smad signalling, whereas receptors endocytosed via membrane rafts are targeted for degradation. The objective of the present study was to evaluate the contribution of TβRIII to TβRII and TβRI membrane partitioning, receptor half-life and signalling. Using sucrose-density ultracentrifugation to isolate membrane-raft fractions, we show that TβRIII recruits both TβRII and TβRI to non-raft membrane fractions. Immunofluorescence microscopy analysis demonstrated that overexpression of TβRIII affects intracellular trafficking of TβRII by recruiting TβRII to EEA1 (early endosome antigen 1)- and Rab5-positive early endosomes. Using 125 I-labelled TGFβ1 to follow cell-surface receptor degradation we show that overexpression of TβRIII also extends the receptor half-life of the TβRII–TβRI complex. Interestingly, we also show, using a luciferase reporter assay, that TβRIII increases basal TGFβ signalling. As numerous pathologies show aberrant activation of TGFβ signalling, the present study illustrates that TβRIII may represent a novel therapeutic target.

Cell-surface TGFβ (transforming growth factor β) receptors partition into membrane rafts and the caveolin-positive endocytic compartment by an unknown mechanism. In the present study, we investigated the determinant in the TGFβ type II receptor (TβRII) that is necessary for membrane raft/caveolar targeting. Using subcellular fractionation and immunofluorescence microscopy techniques, we demonstrated that the extracellular domain of TβRII mediates receptor partitioning into raft and caveolin-positive membrane domains. Pharmacological perturbation of glycosylation using tunicamycin or the mutation of Mgat5 [mannosyl(α-1,6)-glycoprotein β-1,6-N-acetylglucosaminyltransferase V] activity interfered with the raft partitioning of TβRII. However, this was not due to the glycosylation state of TβRII, as a non-glycosylated TβRII mutant remained enriched in membrane rafts. This suggested that other cell-surface glycoproteins associate with the extracellular domain of TβRII and direct their partitioning in membrane raft domains. To test this we analysed a GMCSF (granulocyte/macrophage colony-stimulating factor)–TβRII chimaeric receptor, which contains a glycosylated GMCSF extracellular domain fused to the transmembrane and intracellular domains of TβRII. This chimaeric receptor was found to be largely excluded from membrane rafts and caveolin-positive structures. Our results indicate that the extracellular domain of TβRII mediates receptor partitioning into membrane rafts and efficient entrance into caveolin-positive endosomes.

Receptor-mediated endocytosis and subsequent endosomal proteolysis of [ 125 I]Tyr A14 -[His A8 ,His B4 ,Glu B10 ,His B27 ]insulin ([ 125 I]Tyr A14 -H2 analogue), an insulin analogue exhibiting a high affinity for the insulin receptor, has been studied in liver parenchymal cells by quantitative subcellular fractionation and compared with that of wild-type [ 125 I]Tyr A14 -insulin. Whereas the kinetics of uptake of the H2 analogue by liver was not different from that of insulin, the H2 analogue radioactivity after the 2 min peak declined significantly more slowly. A significant retention of the H2 analogue compared with insulin in both plasma membrane and endosomal fractions was observed and corresponded to decreased processing and dissociation of the H2 analogue. Cell-free endosomes preloaded in vivo with radiolabelled ligands and incubated in vitro processed insulin and extraluminally released insulin intermediates at a 2–3-fold higher rate than the H2 analogue. In vitro proteolysis of both non-radiolabelled and monoiodinated molecules by endosomal lysates showed a decreased response to the endosomal proteolytic machinery for the H2 analogue. However, in cross-linking and competition studies the H2 analogue exhibited an affinity for insulin-degrading enzyme identical with that of wild-type insulin. Brij-35-permeabilized endosomes revealed a 2-fold higher rate of dissociation of insulin from internalized receptors compared with the H2 analogue. After the administration of a saturating dose of both ligands, a rapid and reversible ligand-induced translocation of insulin receptor was observed, but without receptor loss. The H2 analogue induced a higher receptor concentration and tyrosine autophosphorylation of the receptor β subunit in endosomes. Moreover, a prolonged temporal interaction of the in vivo injected H2 analogue with receptor was observed by direct binding assays performed on freshly prepared subcellular fractions. These results indicate that endosomal proteolysis for the H2 analogue is slowed as a result of an increased residence time of the analogue on the insulin receptor and a low affinity of endosomal acidic insulinase for the dissociated H2 molecule.