Science paper identified two proteins that combine to form pentamers, which along with previously discovered hexamers, create a preliminary atomic model.

Researchers identified proteins that play a critical role in how the carboxysome folds. The carboxysome is a protein shell that is one of the most studied microcompartments, which seems to be present in a wide variety of pathogens including bacteria.

Previously, researchers discovered the 3-D structures of hexameric carboxysome shell proteins that make up part of the carboxysome. In the current study, the investigators identified the 3-D structures of two proteins of previously unknown function, CcmL and OrfA (or CsoS4A), that assemble to form pentameric structures of the microcompartment. “Combining these pentamers with the hexamers previously elucidated gives two plausible, preliminary atomic models for the carboxysome shell,” wrote the investigators.

“A soccer ball has hexagons and 12 pentagons at the corners; the pentagons are essential to close the structure,” explains coauthor Todd O. Yeates, Ph.D., professor of chemistry and biochemistry at the University of California at Los Angeles (UCLA). “This principle of closing a structure by combining a large number of hexagons with a small number of pentagons to create a piece of curvature is apparently understood by microcompartments, in which proteins form 12 pentagons to close the structure.” 

The research was performed by scientists at UCLA, the DOE Joint Genome Institute, University of California, Berkeley, and University of Southern Mississippi. The paper is published in the February 22 issue of Science.