Zipper Membranes
New Possibilities for Biological Research

They were looking for a way to define the function of Golgi bodies in plant cells, but along the way, they found something else. A team of scientists from The University of Arizona and the University of Colorado, Boulder, has produced a new tool for biological research and biotechnology. Called “zipper-membranes,” the scientists have discovered a way to genetically engineer membrane organelles inside cells in a way that stimulates the production of foreign proteins in cells without disrupting cell functions. The zipper-membranes store these proteins away from the rest of the cell’s contents, where they can be used in various pharmaceutical, biological and agricultural applications.

David Galbraith, a professor in the Department of Plant Sciences, UA College of Agriculture, with graduate student Fang-Cheng Gong, performed the laboratory experiments leading to the discovery. Scientists from the Department of Molecular and Cellular Biology at the University of Colorado did the electron microscopy needed to examine the engineered cells.

“In producing this protein, we are altering the organelles within the cell to form the zipper-membranes,” Galbraith says. “This represents a new concept.” No one has created an artificial organelle within cells before, although scientists have been able to manufacture foreign proteins in cells using other methods.

"We found that the protien was localized exclusively in scroll-like membranes that looked rather like onions in the cell."These previous attempts to produce foreign proteins in engineered hosts have been hampered by the instability of the proteins, or by their toxicity to the host organism. In contrast, the zipper-membranes accumulate and hold the new proteins in a “stable and presumably active form,” according to the researchers.

The discovery occurred unexpectedly while the scientists were exploring the function of Golgi apparatus, an organelle found in all cells but poorly understood in plants. To learn more about the way the plant Golgi is organized, Galbraith and his associates aimed to insert a protein marker for the Golgi into a plant cell and track its activity.

For this Golgi marker, they used avian infectious bronchitis virus (IBV M), a protein used for identifying Golgi functions in animal cells. To help track the IBV M protein inside the plant cells, they combined it with a bacterial enzyme, beta-glucuronidase (GUS) by making a chimeric gene, one containing the genetic material from both the IBV M gene and the GUS gene.

Inside the tobacco cell, the production of the hybrid protein enacted a startling process that led to the formation of an entirely new membrane organelle, the zipper-membranes.

“When we looked at the cells through the electron microscope in Colorado, we found that the protein was localized exclusively in scroll-like membranes that looked rather like onions in the cell,” Galbraith says. “The membranes were accumulating and somehow rolling up on themselves.”

Production of the hybrid protein had caused the cell to generate masses of membranes similar to those found in the endoplasmic reticulum, a network of tubular membranes normally responsible for transporting materials within the cell. However, in this case, the membranes had folded and compressed into whorls, or zippered themselves together, hence the choice of the name zipper-membranes, or z-membranes.

“It occurred to us that making these proteins inside the z-membranes protects them from degradation,” Galbraith explains. From the biotechnological point of view this is important, since foreign proteins made inside host cells are often held back in the endoplasmic reticulum, and broken down. Instead, this specialized endoplasmic reticulum membrane had allowed the overproduction and storage of the foreign protein.

“If you wanted to accumulate a protein that would be of biological use, this system might allow you to produce a lot of these proteins,” he says. Antigens for oral vaccines are one possibility; membrane enzymes for processing biological molecules are another. The method lends itself to applications in biochemical and crystallographic studies, and in biosensor systems.

“The plants look entirely normal,” Galbraith adds. “This means the cells tolerate the production of the z-membranes. We think that this should allow us to produce many different types of protein inside the zippered membranes without injuring the cell.

“You could use the z-membrane system to accumulate proteins that are toxic to an insect pest but not the plant. This accumulated protein might even be a substance that would normally kill the plant.”

In recent studies, the researchers have found the same protein induced formation of z-membranes in other types of organisms, not just plants.

The University of Arizona and the University of Colorado, Boulder, have applied for a joint patent on the process, and the results of this research have been published in the March 1996 issue of the Proceedings of the National Academy of Sciences.


Article Written by Susan McGinley, ECAT, College of Agriculture
This is part of the 1996 Arizona Experiment Station Research Report
This document is located at http://ag.arizona.edu/pubs/general/resrpt1996/zipper.html
Return to index for 1996 report

Researcher:

David Galbraith, Department of Plant Sciences
Phone: (520) 621-9153
dgalbrai@ag.arizona.edu