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Actobindin is an 88-amino acid polypeptide, containing two almost identical repeated domains of 33 and 34 residues. Depending on the molar ratios in which they are mixed, actobindin binds either one or two actin molecules. We cross-linked actobindin and actin in the 1:1 complex, using the zero-length cross-linker 1-ethyl-3(3-dimethylaminopropyl)carbodiimide. The cross-linked peptides were purified after consecutive CNBr cleavage and trypsin and Staphylococcus protease V8 digestions, and the cross-linked side chains were identified by amino acid sequencing. Isopeptide linkages were formed between residues Glu-100 of actin and Lys-16 of actobindin. In addition, we found a connection between one or more of the acidic residues 1,2, or 3 of actin and Lys-16 and Lys-52 of actobindin. The cross-linked regions in actobindin contain Leu-Lys-His-Ala-Glu-Thr motifs, similar to sequences observed in several other actin-binding proteins.

Actobindin is a protein from Acanthamoeba castellanii with bivalent affinity for monomeric actin. Because it can bind two molecules of actin, actobindin is a substantially more potent inhibitor of the early phase of actin polymerization than of F-actin elongation. The complete amino acid sequence of 88 residues has been deduced from the determined sequences of overlapping peptides obtained by cleavage with trypsin, Staphylococcus V8 protease, endoproteinase Asp-N, and CNBr. Actobindin contains 2 trimethyllysine residues and an acetylated NH2 terminus. About 76% of the actobindin molecule consists of two nearly identical repeated segments of approximately 33 residues each. This could explain actobindin's bivalent affinity for actin. The circular dichroism spectrum of actobindin is consistent with 15% alpha-helix and 22% beta-sheet structure. A hexapeptide with sequence LKHAET, which occurs at the beginning of each of the repeated segments of actobindin, is very similar to sequences found in tropomyosin, muscle myosin heavy chain, paramyosin, and Dictyostelium alpha-actinin. A longer stretch in each repeated segment is similar to sequences in mammalian and amoeba profilins. Interestingly, the sequences around the trimethyllysine residues in each of the repeats are similar to the sequences flanking the trimethyllysine residue of rabbit reticulocyte elongation factor 1 alpha, but not to the sequences around the trimethyllysine residues in Acanthamoeba actin and Acanthamoeba profilins I and II.