Actin: an essential player in cell adhesion and migration

Found in all eukaryotic cells, actin is a protein essential to maintain cell structure. In humans, three main actin groups have been identified - alpha, beta, and gamma.

The alpha actins are expressed primarily in skeletal, cardiac, and smooth muscle, while the remaining beta and gamma. Actin filaments provide internal mechanical support to maintain cell shape, as well as the force to drive cell movements and intracellular transport. Actin filaments are also involved in coordinating the movements of organelles such as mitochondria, Golgi vesicles or peroxisomes, controlling cell-to-cell junctions, and cell polarity¹.

These activities can be extremely complex depending on highly regulated interactions between actin and numerous other ².

Nuclear versus cytoplasmic actin

In the cytoplasm, actin polymerizes in an ATP-dependent manner, into long stable filaments twisted around each other to increase strength. The initial phase is slow, with these actin-ATP complexes aggregating into short and unstable oligomers. Once these oligomers reach a certain length they can act as a stable seed or nucleus, which then rapidly elongate to form a filament by the addition of more actin-ATP at both ends.

Once the filament is formed, bound ATP can be hydrolyzed to ADP, which occurs in the middle of the filament, always leaving the ends with actin-ATP. However, this hydrolysis is not essential for the formation of filaments, as shown by studies using non-hydrolyzable ATP analogs, where actin is still able to polymerize into filaments³.

Regulatory control of subtle changes in the ATP-actin filaments and the activity of actin involve over 100 regulatory proteins. These proteins act to restrict the length of filaments, regulate assembly and turnover; as well as stimulating the formation of cross-link filaments into bundles^2,4.

In addition to cytoplasmic functions, recent findings place actin in the nucleus. For many years, its nuclear presence was considered an artifact, but researchers have discovered that actin is involved in chromatin remodeling and transcriptional regulation⁵.

For example, nuclear actin is an important component of several chromatin-altering machines by binding together with chromatin remodelers (eg SWI-SNF, SWR1 and INO80) or modifiers (eg NuA4 HAT)⁶. Actin also appears to participate in transcriptional initiation via activity in pre-messenger RNA and interactions with RNA polymerase I, II and III⁷.

Actin as a loading control

Interestingly, each actin isoform is remarkably similar to all other isoforms, with minor variations in the amino acid sequence. Beta cyto-actin and gamma cyto-actin, for example, only differ by four residues, suggesting a strong evolutionary pressure to maintain these small sequence differences¹. Such homology information among the actin members and across species is important for antibody selection and the interpretation of western blot bands.

Traditionally, beta actin has been the most common loading control in biomedical research as it is generally accepted that protein levels do not vary significantly due to developmental stage, composition of cell types, or sample treatments. However, recent studies have started to question the validity of this practice.

For example, expression of beta-actin was shown to vary with tissue maturation and researchers now advise against the use of beta actin in experiments, particularly when a long range of developmental stages or different tissues are analyzed^8,9.