Actin is one of the most abundant proteins in eukaryotic cells, often accounting for up to 10 percent of total protein. In muscles, it helps generate force for movement. In virtually all other cells it is part of the structure that gives cells their shape, helps cells to move, and is also involved in chromosome segregation during cell division. Actin forms filaments that are bundled and cross-linked by α-actinin, an important actin-binding protein. „α-Actinin comes in two ‘flavors’ or isoforms”, explains group leader Kristina Djinovic-Carugo, „one is regulated by PIP2 and is found in muscles. The other form is regulated by calcium.” It was already known that high concentrations of calcium inhibit the ability of non-muscle α-actinin to bundle actin filaments. The mechanism behind this regulation, however, was unclear.
The scientists have now determined the molecular structures of an ancestral α-actinin from the parasite E. histolytica in the presence and absence of calcium by X-ray crystallography. They found that a short section of the protein, the so-called calmodulin-like domain, exhibits a very high affinity for calcium. “Binding of calcium to this domain causes other domains of the protein to rigidify”, explains Kristina Djinovic-Carugo. The individual domains interlock with each other similar to joints. Ultimately, the distal “head” part of α-actinin, which binds to actin filaments, loses its flexibility and so its ability to bundle actin filaments.
Together with their collaborators from the Jawaharlal Nehru University the team of researchers also shows the importance of α-actinin in the survival of the parasite E. histolytica. In order to propagate, the parasite encapsulates other cells or bacteria in tissues of the host organism, a process called phagocytosis. “Mutant E. histolytica α-actinin that can’t bind calcium can’t complete the phagocytotic cycle”, says Kristina Djinovic-Carugo. “α-Actinin therefore might represent a new drug target for amoebic dysentery”. In addition, E. histolytica α-actinin shares a highly similar architecture with human α-actinin. This study may therefore also help to understand α-actinin regulation in higher organisms. In the next step, the scientists hope to determine the structure of a human calcium-regulated α-actinin to better understand the role of calcium in α-actinin regulation.
Nikos Pinotsis, Karolina Zielinska, Mrigya Babuta, Joan L. Arolas, Julius Kostan, Muhammad Bashir Khan, Claudia Schreiner, Anita Salmazo, Luciano Ciccarelli, Martin Puchinger, Eirini A. Gkougkoulia, Euripedes de Almeida Ribeiro, Thomas C. Marlovits, Alok Bhattacharya & Kristina Djinovic-Carugo: Calcium modulates the domain flexibility and function of an α-actinin similar to the ancestral α-actinin. Proceedings of the National Academy of Sciences of the United States of America (PNAS), 2020