[Mechanisms of spatial segregation of actin isoforms]

Actin sequences are conserved to a much greater degree than those in almost any other proteins, so that two cytoplasmic isoforms differ by only four of 374 amino acid residues. Nevertheless, the results of biochemical, immunocytochemical and molecular biology experiments demonstrate that appearance, amount and localization of actin isoforms are strongly controlled by cell machinery. Although at the early stages of cell differentiation expression of any actin gene is potentially possible, under normal physiological conditions, while differentiation proceeds, synthesis of specific actin isoforms is temporally regulated and the produced proteins are segregated spatially. Pathological situations of tissue injury or mammalian disease correlate either with up- and down-regulation of distinct actin genes returning to a fetal gene program or with a failure to sort actin isoforms. Different actin isoforms cannot substitute for each other, and changes in expression of specific actin genes are accompanied by alterations in cell structure and function suggesting that specific actin isoforms perform unique cellular functions. This article summarizes the data on segregation of actin isoforms in cell compartments and analyses the mechanisms suggested to explain spatial segregation of cytoplasmic actin isoforms within a cell.