25.10: Introduction to Actin
Actin is a highly conserved cytoskeletal protein found abundantly in eukaryotic cells. It constitutes 10% weight of the total cellular protein in muscle cells, while in non-muscle cells, it is lower and makes up around 1–5 percent of the total cell protein. Actin found in the unicellular amoebae and complex multicellular animals is around 80% similar, demonstrating their conservation over a billion years of evolution. Actin coding genes are conserved within species and across different species. For example, actins present in the yeast Saccharomyces cerevisiae and humans are 87% similar.
Actin was first discovered by W. D. Halliburton in 1887 in muscle extracts as a protein that can induce coagulation of the muscle plasma. It was later purified from muscle extract by Brunó Ferenc Straub in 1942. Straub named it 'actin' due to its ability to activate the motor protein myosin. In the early seventies, actin was also found in non-muscle cells and Acanthamoeba. In the late 1970s, Multiple actin isoforms with issue-specific expression were discovered by sequencing actins using amino acid hydrolysis. These isoforms were nearly identical, with only a few amino acid substitutions. They were classified into α-, β-, and γ-actins based on their isoelectric points. For example, birds and mammals have six actin isoforms expressed at different cell types. Actins expressed in skeletal muscle cells are classified as α-skeletal-actin, in cardiac muscles as α-cardiac-actin, and those in smooth muscles as α-smooth-actin and γ-smooth-actin. Two other isoforms, β-cyto-actin and γ-cyto-actin, are ubiquitously expressed in these organisms.
Subsequent research on these proteins led to the discovery of their role in various cellular functions such as muscle contraction, cell migration, cell adhesion, cell division, protein trafficking, and membrane organization. The first X-ray structure of monomeric G-actin associated with DNAse I was solved only in 1990, resulting in the first atomic model of actin filaments. Several other structures in complex with different proteins have been reported since then.
