Adaptor protein complexes as the key regulators of protein sorting in the post-Golgi network.
Adaptor protein (AP) complexes are cytosolic heterotetramers that mediate the sorting of membrane proteins in the secretory and endocytic pathways. AP complexes are involved in the formation of clathrin-coated vesicles (CCVs) by recruiting the scaffold protein, clathrin. AP complexes also play a pivotal role in the cargo selection by recognizing the sorting signals within the cytoplasmic tail of integral membrane proteins. Six distinct AP complexes have been identified. AP-2 mediates endocytosis from the plasma membrane, while AP-1, AP-3 and AP-4 play a role in the endosomal/lysosomal sorting pathways. Moreover, tissue-specific sorting events such as the basolateral sorting in polarized epithelial cells and the biogenesis of specialized organelles including melanosomes and synaptic vesicles are also regulated by members of AP complexes. The application of a variety of methodologies have gradually revealed the physiological role of AP complexes. (+info)
Crystal structure of the clathrin adaptor protein 1 core.
The heterotetrameric adaptor proteins (AP complexes) link the outer lattice of clathrin-coated vesicles with membrane-anchored cargo molecules. We report the crystal structure of the core of the AP-1 complex, which functions in the trans-Golgi network (TGN). Packing of complexes in the crystal generates an exceptionally long (1,135-A) unit-cell axis, but the 6-fold noncrystallographic redundancy yields an excellent map at 4-A resolution. The AP-1 core comprises N-terminal fragments of the two large chains, beta1 and gamma, and the intact medium and small chains, micro1 and sigma1. Its molecular architecture closely resembles that of the core of AP-2, the plasma-membrane-specific adaptor, for which a structure has been determined. Both structures represent an "inactive" conformation with respect to binding of cargo with a tyrosine-based sorting signal. TGN localization of AP-1 depends on the small GTPase, Arf1, and the phosphoinositide, PI-4-P. We show that directed mutations of residues at a particular corner of the gamma chain prevent recruitment to the TGN in cells and diminish PI-4-P-dependent, but not Arf1-dependent, liposome binding in vitro. (+info)
The ubiquitously expressed Csk adaptor protein Cbp is dispensable for embryogenesis and T-cell development and function.
Regulation of Src family kinase (SFK) activity is indispensable for a functional immune system and embryogenesis. The activity of SFKs is inhibited by the presence of the carboxy-terminal Src kinase (Csk) at the cell membrane. Thus, recruitment of cytosolic Csk to the membrane-associated SFKs is crucial for its regulatory function. Previous studies utilizing in vitro and transgenic models suggested that the Csk-binding protein (Cbp), also known as phosphoprotein associated with glycosphingolipid microdomains (PAG), is the membrane adaptor for Csk. However, loss-of-function genetic evidence to support this notion was lacking. Herein, we demonstrate that the targeted disruption of the cbp gene in mice has no effect on embryogenesis, thymic development, or T-cell functions in vivo. Moreover, recruitment of Csk to the specialized membrane compartment of "lipid rafts" is not impaired by Cbp deficiency. Our results indicate that Cbp is dispensable for the recruitment of Csk to the membrane and that another Csk adaptor, yet to be discovered, compensates for the loss of Cbp. (+info)
Tetanus toxin is internalized by a sequential clathrin-dependent mechanism initiated within lipid microdomains and independent of epsin1.
Ligand-receptor complexes are internalized by a variety of endocytic mechanisms. Some are initiated within clathrin-coated membranes, whereas others involve lipid microdomains of the plasma membrane. In neurons, where alternative targeting to short- or long-range trafficking routes underpins the differential processing of synaptic vesicle components and neurotrophin receptors, the mechanism giving access to the axonal retrograde pathway remains unknown. To investigate this sorting process, we examined the internalization of a tetanus neurotoxin fragment (TeNT HC), which shares axonal carriers with neurotrophins and their receptors. Previous studies have shown that the TeNT HC receptor, which comprises polysialogangliosides, resides in lipid microdomains. We demonstrate that TeNT HC internalization also relies on a specialized clathrin-mediated pathway, which is independent of synaptic vesicle recycling. Moreover, unlike transferrin uptake, this AP-2-dependent process is independent of epsin1. These findings identify a pathway for TeNT, beginning with the binding to a lipid raft component (GD1b) and followed by dissociation from GD1b as the toxin internalizes via a clathrin-mediated mechanism using a specific subset of adaptor proteins. (+info)
Long double-stranded RNA produces specific gene downregulation in Giardia lamblia.