A human red blood cell is a dimpled ballerina, ceaselessly spinning, tumbling, bending, and squeezing through openings narrower than its width to dispense life-giving oxygen to every corner of the body. In a paper published in the October issue of Annals of Biomedical Engineering, which was made available online on Oct. 21, a team of UCSD researchers describe a mathematical model that explains how a mesh-like protein skeleton gives a healthy human red blood cell both its rubbery ability to stretch without breaking, and a potential mechanism to facilitate diffusion of oxygen across its membrane.
“Red cells are one of the few kinds of cells in the body with no nucleus and only a thin layer of protein skeleton under their membrane: they are living bags of hemoglobin,” said Amy Sung, a professor of bioengineering at UCSD’s Jacobs School of Engineering and coauthor of the study. “Very little is known about how the elements of the membrane skeleton behave when red blood cells deform, and we were amazed at what our simulation revealed.” Scientists have been mystified for years by the human red blood cell membrane skeleton, a network of roughly 33,000 protein hexagons that looks like a microscopic geodesic dome. Unfortunately, neither the architecture of the dome nor the structures of individual proteins that make up the hexagons reveal the details of how the remarkably regular organization actually works.
Sung and her collaborators at the Jacobs School of Engineering focused on what they view is a key component at the center of each hexagon, a rod-shaped protein complex called the proto-filament. The proto-filament is 37 nanometers in length and made of a protein called actin. Elsewhere in the human body, bundles of actin form contractile muscles, and matrices of actin are responsible for the gel-like properties of various cells’ cytoplasm. However, the foreshortened actin fibers in the proto-filaments act as rigid rods held in suspension by six precisely positioned fibers made of the actin-binding protein spectrin.
Rex Graham | EurekAlert!
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Proteins must be folded correctly to fulfill their molecular functions in cells. Molecular assistants called chaperones help proteins exploit their inbuilt folding potential and reach the correct three-dimensional structure. Researchers at the Max Planck Institute of Biochemistry (MPIB) have demonstrated that actin, the most abundant protein in higher developed cells, does not have the inbuilt potential to fold and instead requires special assistance to fold into its active state. The chaperone TRiC uses a previously undescribed mechanism to perform actin folding. The study was recently published in the journal Cell.
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