Clathrin complex

Abstract: Aggregation of the high-affinity immunoglobulin E (IgE) receptor (FcepsilonRI), expressed on mast cells and basophils, initiates the immediate hypersensitivity reaction. Aggregated FcepsilonRI has been reported to rapidly migrate to lipid rafts in RBL-2H3 cells. We confirmed that aggregated FcepsilonRI is found in the lipid raft fractions of cellular lysates. Furthermore, we show that the cross-linked FcepsilonRI remains associated with detergent-resistant structures upon internalization. Previous morphological studies have reported that aggregated FepsiloncRI is endocytosed via clathrin-coated pits, which in general are not lipid raft associated. To address this apparent discrepancy, we employed siRNA to suppress expression of components of the clathrin-mediated internalization machinery, namely, clathrin heavy chain, and the AP-2 (alpha-adaptin or mu2-subunit). Transferrin receptor (TfR) is endocytosed by a clathrin-mediated process and, as expected, each transfected siRNA caused a two to threefold elevation of TfR surface expression and almost completely inhibited its endocytosis. In contrast, there was no effect on surface expression levels of FcepsilonRI nor on the endocytosis of the dinitrophenyl-human serum albumin (DNP-HSA)/IgE/FcepsilonRI complex. On the contrary, internalization of DNP-HSA/IgE/FcepsilonRI was inhibited by overexpression of a dominant-negative dynamin mutant. We conclude that internalization of cross-linked FcRI does not require the AP-2/clathrin complex but is dynamin-dependent and may be lipid raft mediated.

Abstract: TACC3 regulates spindle organization during mitosis and also regulates centrosome-mediated microtubule nucleation by affecting γ-Tubulin ring complexes. In addition, it interacts with different proteins (such as ch-TOG, clathrin and Aurora-A) to function in mitotic spindle assembly and stability. By forming the TACC3/ch-TOG complex, TACC3 acts as a plus end-tracking protein to promote microtubule elongation. The TACC3/ch-TOG/clathrin complex is formed to stabilize kinetochore fibers by crosslinking adjacent microtubules. Furthermore, the phosphorylation of TACC3 by Aurora-A is important for the formation of TACC3/ch-TOG/clathrin and its recruitment to kinetochore fibers. Recently, the aberrant expression of TACC3 in a variety of human cancers has been linked with mitotic defects. Thus, in this review, we will discuss our current understanding of the biological roles of TACC3 in mitotic spindle organization.

Abstract: Obesity and type 2 diabetes are risk factors of Alzheimer’s disease (AD). We reported that a high fat diet (HFD) promotes amyloid precursor protein (APP) cleavage by β-site APP cleaving enzyme 1 (BACE1) without increasing BACE1 levels in APP transgenic mice. However, the detailed mechanism had remained unclear. Here we demonstrate that HFD promotes BACE1/Adaptor protein-2 (AP-2)/clathrin complex formation by increasing AP-2 levels in APP transgenic mice. In Swedish APP overexpressing Chinese hamster ovary (CHO) cells as well as in SH-SY5Y cells, overexpression of AP-2 promoted the formation of BACE1/AP-2/clathrin complex, increasing the level of the soluble form of APP β (sAPPβ). On the other hand, mutant D495R BACE1, which inhibits formation of this trimeric complex, was shown to decrease the level of sAPPβ. Overexpression of AP-2 promoted the internalization of BACE1 from the cell surface, thus reducing the cell surface BACE1 level. As such, we concluded that HFD may induce the formation of the BACE1/AP-2/clathrin complex, which is followed by its transport of BACE1 from the cell surface to the intracellular compartments. These events might be associated with the enhancement of β-site cleavage of APP in APP transgenic mice. Here we present evidence that HFD, by regulation of subcellular trafficking of BACE1, promotes APP cleavage.