The amino acid sequence of Physarum actin

Abstract

THE realisation that actin is involved not only in contractility in muscle tissue but also in general cytoplasmic movement and cell locomotion was based on the isolation of actin from lower eukaryotic organisms, such as Physarum polycephalum¹, which show active protoplasmic streaming. It is now accepted that actin is the major structural protein of microfilaments characteristically found in the cytoplasm of most if not all higher cells, and that actins have been highly conserved during eukaryotic evolution and differentiation due to the constraints imposed by the large number of specific interactions which actin shows with other cytoplasmic proteins (for a review see ref. 2). Preliminary fingerprint analysis on Physarum actin³ and partial amino acid sequence analysis on actin from Acanthamoeba (ref. 4 and Elzinga cited in ref. 2) have emphasised this point, although so far a complete amino acid sequence has not been reported for actin from a lower eukaryotic species. Our studies on the primary structure of different mammalian actins have shown that several cytoplasmic actins are very similar if not identical in amino acid sequence, although they differ by at least 25 amino acids from the corresponding skeletal muscle actin⁵. The availability of fast amino acid sequence procedures for actin⁵ led us to study the actin from Physarum polycephalum to assess how cytoplasmic actins differ between lower and higher eukaryotes. We report here results which show that Physarum plasmodia, in contrast to non-muscle cells of higher vertebrates^5–7, contain only one cytoplasmic actin species. This actin is more closely related to mammalian cytoplasmic actins than these actins are to skeletal muscle actin. The high degree of structural conservation of the actin sequence and structure during eukaryotic evolution is illustrated by the absence of any amino acid deletions or additions to the polypeptide chain beyond residue 2. Furthermore, the distribution of the proline residues remains unchanged and substitutions of amino acid residues with charged side chains are very rare, indicating a high conservation of the surface topography of the actin molecule.