Structural characterization of coatomer in its cytosolic state

Studies on coat protein I (COPI) have contributed to a basic understanding of how coat proteins generate vesicles to initiate intracellular transport. The core component of the COPI complex is coatomer, which is a multimeric complex that needs to be recruited from the cytosol to membrane in order to function in membrane bending and cargo sorting. Previous structural studies on the clathrin adaptors have found that membrane recruitment induces a large conformational change in promoting their role in cargo sorting. Here, pursuing negative-stain electron microscopy coupled with single-particle analyses, and also performing CXMS (chemical cross-linking coupled with mass spectrometry) for validation, we have reconstructed the structure of coatomer in its soluble form. When compared to the previously elucidated structure of coatomer in its membrane-bound form we do not observe a large conformational change. Thus, the result uncovers a key difference between how COPI versus clathrin coats are regulated by membrane recruitment.

Depletion of epsilon-COP in the COPI Vesicular Coat Reduces Cleistothecium Production in Aspergillus nidulans

We have previously isolated epsilon-COP, the alpha-COP interactor in COPI of Aspergillus nidulans, by yeast two-hybrid screening. To understand the function of epsilon-COP, the aneA+ gene for epsilon-COP/AneA was deleted by homologous recombination using a gene-specific disruption cassette. Deletion of the epsilon-COP gene showed no detectable changes in vegetative growth or asexual development, but resulted in decrease in the production of the fruiting body, cleistothecium, under conditions favorable for sexual development. Unlike in the budding yeast Saccharomyces cerevisiae, in A. nidulans, over-expression of epsilon-COP did not rescue the thermo-sensitive growth defect of the alpha-COP mutant at 42degrees C. Together, these data show that epsilon-COP is not essential for viability, but it plays a role in fruiting body formation in A. nidulans.

Genomic Organization of ancop Gene for alpha-COP Homolog from Aspergillus nidulans

We have cloned a alpha-COP homolog, ancop, from Aspergillus nidulans by colony hybridization of chromosome specific library using alpha-COP homologous fragment as a probe. The probe DNA was amplified with degenerated primers designed by comparison of conserved region of the amino acid sequences of Saccharomyces cerevisiae alpha-COP, Homo sapiens HEP-COP, and Drosophila melanogaster alpha-COP. Full length cDNA clone was also amplified by RT-PCR. Comparison of genomic DNA sequence with cDNA sequence obtained by RT-PCR revealed 7 introns. Amino acid sequence similarity search of the anCop with other alpha-COPs gave an overall identity of 52% with S. cerevisiae, 47% with human and bovine, 45% with Drosophila and Arabidopsis . In upstream region from the transcription start site, a putative TATA and CAAT motif were also identified.

Mechanism of formation of post Golgi vesicles from TGN membranes: Arf-dependent coat assembly and PKC-regulated vesicle scission

We have developed an experimental system that utilizes purified Golgi fractions obtained from virus infected infected MDCK cells to reproduce in vitro the process of vesicle generation in the trans Golgi network, an important site for the sorting of proteins addressed to the plasma membrane, secretory vesicles, or lysosomes. Using an integrated biochemical and electron microscopic approach, we have shown that the formation of post Golgi vesicles carrying proteins destined to both plasma membrane domains of epithelial cells requires the activation of an ArF-like GTP-binding protein that serves to promote the assembly of the protein coat necessary to deform the donor membrane and generate a vesicle. The formation of the post Golgi vesicles also requires the participation of a Golgi membrane-associated Protein Kinase C, but not its phosphorylating activity. Other authors have shown that this is also the case for the PKC activation of the enzyme phospholipase D, which generates phosphatidic acid from phosphatidyl choline and may be involved in remodeling of membranes. We have been able to dissect the process of post Golgi vesicle generation into two sequential stages, one of coat assembly and bud formation, and a subsequent one of vesicle scission. The first stage can occur at 20 degrees C and requires the activation of the Arf protein necessary for coat assembly. The second stage does not require nucleotides or an energy supply, but requires cytosolic proteins, and in particular, an NEM sensitive membrane scission promoting activity that operates only at a higher temperature of incubation. Because various PKC inhibitors blocked vesicle scission without preventing bud formation, we propose that the PKC is required for the activation of a PLD in the TGN, which leads to remodeling of the donor membrane and the severing of connections between the emerging vesicles and the membranes.