Cellular internalization of green fluorescent protein fused with herpes simplex virus protein VP22 via a lipid raft-mediated endocytic pathway independent of caveolae and Rho family GTPases but dependent on dynamin and Arf6

VP22 is a structural protein of the herpes simplex virus and has been reported to possess unusual trafficking properties. Here we examined the mechanism of cellular uptake of VP22 using a fusion protein between the C-terminal half of VP22 and green fluorescent protein (GFP). Adsorption of VP22-GFP onto a cell surface required heparan sulfate proteoglycans and basic amino acids, in particular, Arg-164 of VP22. Inhibitor treatment, RNA interference, expression of dominant-negative mutant genes, and confocal microscopy all indicated that VP22-GFP enters cells through an endocytic pathway independent of clathrin and caveolae but dependent on dynamin and Arf6 activity. As with CD59 (a lipid raft marker), cell-surface VP22-GFP signals were resistant to Triton X-100 treatment but only partially overlapped cell-surface CD59 signals. Furthermore, unlike other lipid raft-mediated endocytic pathways, no Rho family GTPase was required for VP22-GFP internalization. Internalized VP22 initially entered early endosomes and then moved to lysosomes and possibly recycling endosomes.     

Carbohydrate-recognition domains of galectin-9 are involved in intermolecular interaction with galectin-9 itself and other members of the galectin family

Galectin-9 (Gal-9) is a tandem-repeat-type member of the galectin family associated with diverse biological processes, such as apoptosis, cell aggregation, and eosinophil chemoattraction. Although the detailed sugar-binding specificity of Gal-9 has been elucidated, molecular mechanisms that underlie these functions remain to be investigated. During the course of our binding study by affinity chromatography and surface plasmon resonance (SPR) analysis, we found that human Gal-9 interacts with immobilized Gal-9 in the protein-protein interaction mode. Interestingly, this intermolecular interaction strongly depended on the activity of the carbohydrate recognition domain (CRD), because the addition of potent saccharide inhibitors abolished the binding. The presence of multimers was also confirmed by Ferguson plot analysis of result of polyacrylamide gel electrophoresis and matrix-assisted laser-desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS). Moreover, this intermolecular interaction was observed between Gal-9 and other galectin members, such as Gal-3 and Gal-8, but not Gal-1. Because such properties have not been reported yet, they may explain an unidentified mechanism underlying the diverse functions of Gal-9.     

Functional regulation of the DNA damage-recognition factor DDB2 by ubiquitination and interaction with xeroderma pigmentosum group C protein

In mammalian nucleotide excision repair, the DDB1–DDB2 complex recognizes UV-induced DNA photolesions and facilitates recruitment of the XPC complex. Upon binding to damaged DNA, the Cullin 4 ubiquitin ligase associated with DDB1–DDB2 is activated and ubiquitinates DDB2 and XPC. The structurally disordered N-terminal tail of DDB2 contains seven lysines identified as major sites for ubiquitination that target the protein for proteasomal degradation; however, the precise biological functions of these modifications remained unknown. By exogenous expression of mutant DDB2 proteins in normal human fibroblasts, here we show that the N-terminal tail of DDB2 is involved in regulation of cellular responses to UV. By striking contrast with behaviors of exogenous DDB2, the endogenous DDB2 protein was stabilized even after UV irradiation as a function of the XPC expression level. Furthermore, XPC competitively suppressed ubiquitination of DDB2 in vitro, and this effect was significantly promoted by centrin-2, which augments the DNA damage-recognition activity of XPC. Based on these findings, we propose that in cells exposed to UV, DDB2 is protected by XPC from ubiquitination and degradation in a stochastic manner; thus XPC allows DDB2 to initiate multiple rounds of repair events, thereby contributing to the persistence of cellular DNA repair capacity.     

Supersaturation-limited and Unlimited Phase Transitions Compete to Produce the Pathway Complexity in Amyloid Fibrillation

Although amyloid fibrils and amorphous aggregates are two types of aggregates formed by denatured proteins, their relationship currently remains unclear. We used β₂-microglobulin (β2m), a protein responsible for dialysis-related amyloidosis, to clarify the mechanism by which proteins form either amyloid fibrils or amorphous aggregates. When ultrasonication was used to accelerate the spontaneous fibrillation of β2m at pH 2.0, the effects observed depended on ultrasonic power; although stronger ultrasonic power effectively accelerated fibrillation, excessively strong ultrasonic power decreased the amount of fibrils formed, as monitored by thioflavin T fluorescence. An analysis of the products formed indicated that excessively strong ultrasonic power generated fibrillar aggregates that retained β-structures but without high efficiency as seeds. On the other hand, when the spontaneous fibrillation of β2m was induced at higher concentrations of NaCl at pH 2.0 with stirring, amorphous aggregates became more dominant than amyloid fibrils. These apparent complexities in fibrillation were explained comprehensively by a competitive mechanism in which supersaturation-limited reactions competed with supersaturation-unlimited reactions. We link the kinetics of protein aggregation and a conformational phase diagram, in which supersaturation played important roles.     

Crystal structure of the carbon monoxide complex of human cytoglobin

Cytoglobin (Cgb) is a vertebrate heme‐containing globin‐protein expressed in a broad range of mammalian tissues. Unlike myoglobin, Cgb displays a hexa‐coordinated (bis‐hystidyl) heme iron atom, having the heme distal His81(E7) residue as the endogenous sixth ligand. In the present study, we crystallized human Cgb in the presence of a reductant Na₂S₂O₄ under a carbon monoxide (CO) atmosphere, and determined the crystal structure at 2.6 Å resolution. The CO ligand occupies the sixth axial position of the heme ferrous iron. Eventually, the imidazole group of His81(E7) is expelled from the sixth position and swings out of the distal heme pocket. The flipping motion of the His81 imidazole group accompanies structural readjustments of some residues (Gln62, Phe63, Gln72, and Ser75) in both the CD‐corner and D‐helix regions of Cgb. On the other hand, no significant structural changes were observed in other Cgb regions, for example, on the proximal side. These structural alterations that occurred as a result of exogenous ligand (CO) binding are clearly different from those observed in other vertebrate hexa‐coordinated globins (mouse neuroglobin, Drosophila melanogaster hemoglobin) and penta‐coordinated sperm whale myoglobin. The present study provides the structural basis for further discussion of the unique ligand‐binding properties of Cgb. Proteins 2011. © 2011 Wiley‐Liss, Inc.