Research involving a protein called BRICK1 is paving the way to a better understanding of cellular growth and development mechanisms that may result in designer plants with increased health benefits, according to Purdue University researchers.

The ability to control the interaction of BRICK1-related proteins may enable scientists to alter plants so they retain more of certain compounds, making them heartier and a more nutritional food source.

BRICK1 (BRK1) also is part of similar developmental processes in humans and other animals, said Dan Szymanski, a Purdue Department of Agronomy geneticist and cell biologist. The protein's job is to generate networks of filaments inside the cell that provide tracks for long-distance transport of molecules and to regulate development and organization of specialized compartments in the cell. BRK1 and its function in cellular-development is part of a protein complex called WAVE.

Szymanski and his team were able to identify BRK1 and determine its function using the laboratory plant Arabidopsis as a model for how the protein behaves in other plant species and in animals. The results of their study are published online in the journal Current Biology and are scheduled to appear in the Tuesday (May 9) issue of the publication.

The new knowledge about BRK1's and WAVE's contributions to plant growth will enable plant geneticists to manipulate plant cell metabolism and growth in novel ways, Szymanski said.

"Plant cells are the world's greatest organic chemists," he said. "At a very low cost, they can generate an amazing diversity of compounds that are important for human nutrition and health.

"A major payoff from plant biotechnology will be the successful engineering of plants that can synthesize, store and accumulate valuable compounds. We are finding that the filament network, called the actin cytoskeleton, is an important player in the storage and accumulation functions in cells."

The research team found that BRK1 and its associated WAVE complex proteins are key cell growth modulators. These proteins also help control and coordinate growth in complex tissues.

BRK1 stabilizes and prevents degradation of another WAVE complex protein, SCAR2, Szymanski said. Stabilized SCAR2 then is able to activate machinery that controls actin filament formation, which is integral to cell development. Without the SCAR2-like proteins, animals die. But Arabidopsis does not.

"The beauty of the Arabidopsis system is the genes and proteins of the WAVE complex are preserved, but they are not essential for viability of this particular species," Szymanski said. "This allows us to use genetics and biochemistry to discover important pathways that control cells' internal structure, shape and growth."

In both plants and animals, the WAVE complex regulates the timing and location in which SCAR is active. This triggers formation of new actin filaments. The research team found that without BRK1, SCAR proteins degrade quickly, leading to a loss of important actin filaments.

Although many aspects of the functions of actin filament in plants and animals are unclear, some of the jobs are known. Actin filaments form a cytoskeleton that acts as a scaffold to position important intracellular structures. Actin filaments also serve as long-distance tracks to transport materials and structures necessary to control cell shape, growth and movement.

"Actin filaments may not have exactly the same roles in all species of plants and animals, but much of the machinery that controls where and when actin filaments form appears to be the same in many species." Szymanski said.

The next steps in the team's cell development research will be to determine where in the cell BRK1-generated actin filaments are located and their precise function, he said. The scientists believe this will aid in discovering how filaments transport growth material in and out of cellular storage areas.

The other researchers on this study were postdoctoral students Jie Le and Chunhua Zhang, and laboratory manager Eileen L Mallery, all of the Department of Agronomy, and Steven Brankle, a Department of Animal Sciences undergraduate student. Szymanski also is a member of the Purdue Cytoskeleton Group.

The National Science Foundation's Integrated Biology and Neuroscience Division and the Department of Energy's Energy Biosciences Division provided funding for this research.

Writer: Susan A. Steeves
Sources: Dan Szymanski

Related Web sites:

Purdue Department of Agronomy: http://www.agry.purdue.edu/
Purdue Motility Group: http://www.biology.purdue.edu/research/groups/motility/index.htm
Current Biology: http://www.current-biology.com/
Department of Energy Office of Science: http://www.sc.doe.gov/
National Science Foundation: http://www.nsf.gov/

ABSTRACT

Arabidopsis BRICK1/HSPC300 is an essential subunit of the WAVE complex that selectively stabilizes the Arp2/3 activator SCAR2
Jie Le; Eileen L Mallery; Chunhua Zhang; Steven Brankle; Daniel Szymanski

The actin cytoskeleton dynamically reorganizes the cytosplasm during cell morphogenesis. An important goal is to understand mechanisms by which actin filament nucleation is integrated with the cellular growth machinery. The actin related protein (Arp2/3) complex is a potent nucleator of actin filaments that controls a variety of
endomembrane functions including the endocytic internalization of plasma membrane [1], vacuole biogenesis [2, 3], plasma membrane protrusion in crawling cells [4], and membrane trafficking from the Golgi [5]. Therefore, Arp2/3 is an important signaling target during morphogenesis. The Rac-WAVE-Arp2/3 pathway is an evolutionarily
conserved module that links actin filament nucleation with cell morphogenesis [6-9]. WAVE translates Rac-GTP signals into Arp2/3 activation by regulating the stability and/or localization of the activator subunit Scar/WAVE [8, 10-12]. WAVE is a five subunit complex containing SRA1/PIR121/CYFIP1, NAP1/NAP125, Abi-1/Abi-2, Brick1 (Brk1)/HSPC300, and Scar/WAVE [10, 13]: defining the in vivo function of each subunit is an important step to understand this complicated signaling pathway. Brk1/HSPC300 has been the most recalcitrant WAVE complex protein, and has no known function. In this paper we report Arabidopsis brick1 (brk1) is a member of the "distorted group" of trichome morphology mutants that defines a WAVE- ARP2/3 morphogenesis pathway [14]. The array of Brk1-phenotypes is identical to wave and arp2/3 plants, providing the first strong genetic evidence that BRK1 functions within a WAVE-ARP2/3 pathway. We find that BRK1 is a critical WAVE complex subunit that selectively stabilizes the Arp2/3 activator SCAR2.