A microbiologist is studying the structure and function of bacterial cell walls. In an experiment, group A streptococci are treated with chemicals to solubilize the cell wall. Various cell wall-associated proteins are subsequently extracted. Electron microscopic evaluation of a specific protein shows structural homology with mammalian tropomyosin and myosin. This protein acts as a virulence factor for certain species of the organism. Which of the following is the most likely function of this bacterial cell wall-associated protein?

Question

A microbiologist is studying the structure and function of bacterial cell walls.  In an experiment, group A streptococci are treated with chemicals to solubilize the cell wall.  Various cell wall-associated proteins are subsequently extracted.  Electron microscopic evaluation of a specific protein shows structural homology with mammalian tropomyosin and myosin.  This protein acts as a virulence factor for certain species of the organism.  Which of the following is the most likely function of this bacterial cell wall-associated protein?

Quizplus · Accepted Answer

Gram-positive organisms have cell walls composed primarily of a thick peptidoglycan layer and teichoic acid. However, additional cell wall components are often present that contribute to virulence, motility, or the ability to adhere to epithelial surfaces. For instance, the cell wall of group A streptococcus (also known as Streptococcus pyogenes) is studded with M protein, a virulence factor that inhibits phagocytosis, prevents complement binding, and aids in epithelial attachment. M protein is an alpha-helical coiled-coil protein that shares epitopes and structural homology with other alpha-helical coiled-coil proteins such as tropomyosin and myosin. In some patients, the protective antibodies against M protein that develop in S pyogenes acute infection may cross-react with myosin epitopes in the heart and cause rheumatic carditis. (Choice A) Efflux pumps are generally globular proteins with multiple domains. They reside in the bacterial cell membrane and excrete toxic substances from the interior of the cell. Bacteria can use efflux pumps to generate antibiotic resistance to drugs that operate in the interior of the cell, such as fluoroquinolones and aminoglycosides. (Choices B and C) The bacterial cell wall of S pyogenes provides mechanical support and protects it from osmotic lysis. It is composed primarily of peptidoglycan, which forms a mesh-like, cross-linked polymer of peptides and sugars. (Choice E) Porins in the cell membrane aid in the movement of nutrients into bacterial cells. They are composed of protein strands that group together to form a barrel (beta-barrel). Educational objective: The major virulence factor of Streptococcus pyogenes is M protein, an alpha-helical coiled-coil protein that shares structural homology with tropomyosin and myosin. It extends from the cell wall and prevents phagocytosis, inhibits complement binding, and mediates bacterial adherence. Antibodies against M protein form shortly after acute infection and may cross-react with epitopes on myosin, leading to rheumatic carditis. __________ References: M protein and other surface proteins on streptococci. (http://www.ncbi.nlm.nih.gov/pubmed/26866233)

A Microbiologist Is Studying the Structure and Function of Bacterial