Actins [1,2,3,4] are highly conserved contractile proteins that are present in all eukaryotic cells. In vertebrates there are three groups of actin isoforms: α, β and γ. The α actins are found in muscle tissues and are a major constituent of the contractile apparatus. The β and γ actins co-exists in most cell types as components of the cytoskeleton and as mediators of internal cell motility. In plants [5] there are many isoforms which are probably involved in a variety of functions such as cytoplasmic streaming, cell shape determination, tip growth, graviperception, cell wall deposition, etc.

Actin exists either in a monomeric form (G-actin) or in a polymerized form (F-actin). Each actin monomer can bind a molecule of ATP; when polymerization occurs, the ATP is hydrolyzed.

Actin is a protein of from 374 to 379 amino acid residues. The structure of actin has been highly conserved in the course of evolution.

Recently some divergent actin-like proteins have been identified in several species. These proteins are:

• Centractin (actin-RPV) from mammals, fungi (yeast ACT5, Neurospora crassa ro-4) and Pneumocystis carinii (actin-II). Centractin seems to be a component of a multi-subunit centrosomal complex involved in microtubule based vesicle motility. This subfamily is also known as ARP1.
• ARP2 subfamily which includes chicken ACTL, yeast ACT2, Drosophila 14D, C.elegans actC.
• ARP3 subfamily which includes actin 2 from mammals, Drosophila 66B, yeast ACT4 and fission yeast act2.
• ARP4 subfamily which includes yeast ACT3 and Drosophila 13E.

We developed three signature patterns. The first two are specific to actins and span positions 54 to 64 and 357 to 365. The last signature picks up both actins and the actin-like proteins and corresponds to positions 106 to 118 in actins.