Localization of sperm ligand carbohydrate chains in pig zona pellucida glycoproteins

Neutral N-linked carbohydrate chains from pig ZPB/ZPC mixture are shown to possess sperm ligand activity. Of these complex-type chains, triantennary/tetraantennary chains exhibit the activity stronger than that of diantennary chains. Intact ZPB and ZPC cannot be separated from each other unless acidic N-acetyllactosamine regions of their carbohydrate chains are removed by endo-beta-galactosidase digestion. The endo-beta-galactosidase-digested ZPB and its N-terminal fragment of 111 residues retain the sperm ligand activity. Three glycopeptides, having one Asn residue to which the carbohydrate chain is linked, are obtained by lysylendopeptidase digestion of the heat-solubilized zonae containing intact ZPB, and lysylendopeptidase and chymotryptic digestions of endo-beta-galactosidase-digested ZPB. On the basis of sugar mapping analysis of the N-linked chains from these glycopeptides and comparison with the carbohydrate structures of the main intact neutral N-linked chains of ZPB/ZPC, the triantennary and tetraantennary chains are shown to be localized mainly at Asn220 of ZPB, whereas diantennary chains are present on all the three N-glycosylation sites (Asn203, Asn220 and Asn333). These results suggest that the carbohydrate chains linked to Asn220 of ZPB participate predominantly in sperm-egg binding. ZPC has been shown to support the expression of sperm ligand activity of ZPB. Three glycopeptides, each having one of the N-glycosylation sites, are obtained by tryptic digestion of endo-beta-galactosidase-digested ZPC. Triantennary and tetraantennary chains are found mainly at Asn271 of ZPC, whereas diantennary chains are present at all three N-glycosylation sites (Asn124, Asn146 and Asn271). Thus, the localization of triantennary and tetraantennary chains in ZPC is different from that in ZPB.

The presence of a glycosyl phosphatidylinositol-anchored alpha-mannosidase in boar sperm

alpha-Mannosidase and beta-galactosidase were released from boar sperm into the medium by treatment with calcium ionophore A23187 or by 0.2% Brij-35/2% acetic acid. About half as much alpha-mannosidase activity as that in the acid extract was recovered by digestion with phosphatidylinositol-specific phospholipase C (PI-PLC), whereas the liberation rate of beta-galactosidase treated with PI-PLC was low. These results suggest that some alpha-mannosidase is anchored in the plasma membrane of the acrosomal region by attachment to the lipid phosphatidylinositol and that beta-galactosidase is localized mainly in the acrosome or integrated in the plasma membrane by a spanning stretch of hydrophobic peptides. beta-Galactosidase, which is present as an oligomers in the acid extract of sperm, dissociated into monomers under weakly alkaline conditions; under acidic conditions, the monomers associated again. No pH-sensitive association-dissociation of alpha-mannosidase was observed.

Disulfide formation in bovine zona pellucida glycoproteins during fertilization: evidence for the involvement of cystine cross-linkages in hardening of the zona pellucida

The time for solubilization of the bovine zona pellucida in a hypotonic buffer containing 5% (v/v) beta-mercaptoethanol and 7 mol urea l-1 increased by 10% after fertilization. Coupling with a specific fluorescent thiol probe, monobromobimane (mBBr), was markedly greater in the zona pellucida of ovarian eggs compared with fertilized eggs, indicating that the cysteine residues in the zona pellucida of unfertilized eggs are oxidized to cystines during fertilization. After endo-beta-galactosidase digestion to remove N-acetyllactosamine repeats of the carbohydrate chains, three zona pellucida glycoproteins (ZPA, ZPB and ZPC) coupled with the fluorescent bimane groups were fractionated efficiently by reverse-phase HPLC. Estimation of bimane groups in the three components and SDS-PAGE revealed that intramolecular disulfide bonds in ZPA and intra- and intermolecular disulfide bonds in ZPB were formed during fertilization, but oxidation of cysteine residues in ZPC was low. Specific proteolysis of ZPA during fertilization was also observed. These results indicate that the formation of disulfide linkages together with specific proteolysis result in the construction of a rigid zona pellucida structure, which is responsible for hardening of the zona pellucida.

Localization of neutral N-linked carbohydrate chains in pig zona pellucida glycoprotein ZPC

Zona pellucida, a transparent envelope surrounding the mammalian oocyte, plays important roles in fertilization and consists of three glycoproteins; ZPA, ZPB and ZPC. In pig, neutral complex-type N-linked chains obtained from a ZPB/ZPC mixture possess sperm-binding activity. We have recently reported that among neutral N-linked chains triantennary and tetraantennary chains have a sperm-binding activity stronger than that of diantennary chains. Triantennary and tetraantennary chains are localized at the second of the three N-glycosylation sites of ZPB. In this study, we focused on the localization of neutral N-linked chains in ZPC. ZPB and ZPC can not be separated from each other unless the acidic N-acetyllactosamine regions of their carbohydrate chains are removed by endo-beta-galactosidase digestion. A large part of the acidic N-linked chains becomes neutral by the digestion, but the main neutral N-linked chains are not susceptible to the enzyme. N-glycanase digestion indicated that ZPC has three N-glycosylation sites. Three glycopeptides each containing one of the N-glycosylation sites were obtained by tryptic digestion of ZPC and the N-glycosylation sites were revealed as Asn124, Asn146 and Asn271. The carbohydrate structures of the neutral N-linked chains from each glycopeptide were characterized by two-dimensional sugar mapping analysis taking into consideration the structures of the main, intact neutral N-linked chains of ZPB/ZPC mixture reported previously. Triantennary and tetraantennary chains were found mainly at Asn271 of ZPC, whereas diantennary chains were present at all three N-glycosylation sites. Thus, ZPC has tri-antennary and tetra-antennary chains as well as ZPB, but the localization of the chains is different from that in ZPB.

Localization of carbohydrate chains of pig sperm ligand in the glycoprotein ZPB of egg zona pellucida

The three glycoproteins of pig zona pellucida (ZPA, ZPB and ZPC) can be separated into ZPA and a mixture of ZPB/ZPC by gel-filtration HPLC. We have shown previously that the neutral complex-type N-linked carbohydrate chains obtained from ZPB/ZPC possess sperm-binding activity. Intact ZPB and ZPC cannot be separated from each other unless acidic N-acetyllactosamine regions of their carbohydrate chains are removed by endo-beta-galactosidase digestion. The endo-beta-galactosidase-digested ZPB retains the sperm-binding activity. Recently, we have reported that N-linked carbohydrate chains of N-terminal fragment (residues 137-247) obtained from endo-beta-galactosidase-digested ZPB are involved mainly in sperm binding [Yonezawa, N., Mitsui, S., Kudo, K. & Nakano, M. (1997) Eur. J. Biochem. 248, 86-92]. In this study, we separated the intact neutral N-linked chains from the ZPB/ZPC mixture into diantennary chains and triantennary and tetraantennary chains by affinity chromatography on Concanavalia ensiformis agglutinin. An in vitro competition assay revealed that triantennary and tetraantennary chains possess a sperm-binding activity stronger than that of diantennary chains. Three glycopeptides, having one Asn residue to which the carbohydrate chain is linked, were obtained by lysyl endopeptidase digestion of the heat-solubilized zonae containing intact ZPB and lysyl endopeptidase and chymotrypsin A digestion of endo-beta-galactosidase-digested ZPB. From sugar-mapping analysis of the carbohydrate chains from these glycopeptides and comparison with the carbohydrate structures of the main intact neutral N-linked chains of ZPB/ZPC, the triantennary and tetraantennary chains were shown to be localized mainly at Asn220 of ZPB, and diantennary chains were present on all the three potential residues (Asn203, Asn220 and Asn333). These results suggest that the carbohydrate chains linked to Asn220 of ZPB participate predominantly in sperm-egg binding.

Identification of an N-glycosylated region of pig zona pellucida glycoprotein ZPB that is involved in sperm binding

ZPB, one of the pig zona pellucida glycoproteins, can be purified after removal of sialylated and/or sulfated N-acetylpolylactosamine from the nonreducing region of its carbohydrate chains by digestion with endo-beta-galactosidase. Among the components produced, only ZPB shows sperm-binding activity after the digestion. Recently, we have shown that N-linked carbohydrate chains of endo-beta-galactosidase-digested ZPB (EbetaG-ZPB) are predominantly involved in sperm binding [Yonezawa, N., Aoki, H., Hatanaka, Y. & Nakano, M. (1995) Eur. J. Biochem. 233, 35-41]. In this study, to define the sperm-binding region in EbetaG-ZPB, glycopeptides were purified from lysyl endopeptidase digests of EbetaG-ZPB and analyzed for sperm-binding activity by an in vitro competition assay. The locations of the glycopeptides were determined from partial amino acid sequences, amino acid and sugar composition analyses, and apparent molecular masses after SDS/PAGE. The N-terminal fragment (amino acid residues 137-247), which contains two N-linked carbohydrate chains, showed a significant inhibition of sperm-egg binding. However, the fragment that had one N-linked carbohydrate chain (residues 325-341) and the fragment that had two or three O-linked carbohydrate chains (residues 248-324) did not inhibit sperm-egg binding. Thus, the two N-linked carbohydrate chains in the N-terminal fragment of EbetaG-ZPB are important for sperm binding of pig zona pellucida.

A role of cofilin/destrin in reorganization of actin cytoskeleton in response to stresses and cell stimuli

1. Cofilin is an essential actin-regulating protein widely distributed in all eucaryotes. The structure and function of cofilin are conserved during evolution. 2. Cofilin depolymerizes F-actin in vitro at alkaline pH and severs F-actin in vitro at pH lower than 7.3. Overexpression of cofilin in viable cells induced bundles of actin filaments suggesting that the severing activity rather than the actin-depolymerizing or monomeric actin-sequestering activity is physiologically significant in vivo. 3. The actin bundle formation induced by overexpression of cofilin is accompanied with an increase in cell motility of Dictyostelium cells. 4. In higher vertebrates, the actin-binding activity of cofilin is negatively regulated by phosphorylation on its Ser-3 residue. The actin-binding activity is essential for yeast cells to grow. 5. Stresses and various cell stimuli activate cofilin by inducing dephosphorylation of cofilin in resting vertebrate cells. 6. Cofilin has an nuclear localization signal sequence and translocates into the nucleus together with actin in response to various stresses. Functional roles of cofilin/actin in the nucleus remain to be elucidated. 7. Tertiary structure of destrin (cofilin) resembles that of gelsolin segment 1 and well explains its functions such as Ca(2+)-independent actin binding activity.

Structural characterization of the N-linked carbohydrate chains of the zona pellucida glycoproteins from bovine ovarian and fertilized eggs

The N-linked oligosaccharides that were released by hydrazinolysis from glycoproteins of zonae pellucidae of bovine ovarian eggs, were composed of neutral (23%) and acidic (77%) carbohydrate chains; almost all the acidic chains were neutralized by sialidase digestion. Sugar mapping analysis of pyridylaminated N-linked chains by reverse-phase and normal-phase HPLC and 500-MHz 1H-NMR spectroscopy revealed that the major neutral chain is a high-mannose-type oligosaccharide and the acidic chains are di-, tri-, and tetra-antennary, fucosylated complex-type chains that have N-acetyllactosamine repeats in the non-reducing regions. The structures of the neutral chain and the core regions of the acidic chains of N-linked oligosaccharides from the zona proteins of fertilized eggs, which were obtained by the in vitro fertilization method, were essentially the same as those of the ovarian egg zonae. The amount, however, of the acidic chains decreased to 32 mol/100 mol in the fertilized egg zonae, which suggests that a sialidase released from the oocyte during fertilization operates on the zona glycoproteins.

Involvement of N-linked carbohydrate chains of pig zona pellucida in sperm-egg binding

The sperm receptor activity of pig zona pellucida has been previously shown to exist in one of the components, pig zona protein 3 alpha (PZP3 alpha), that can be purified after the removal of sialylated and/or sulfated N-acetylpoly(lactosamine) by digestion with endo-beta-galactosidase. In this study, we examined whether N-linked or O-linked carbohydrate chains are involved in the sperm receptor activity of pig zona pellucida. The elimination of N-linked carbohydrate chains from endo-beta-galactosidase-digested PZP3 alpha by digestion with N-glycanase markedly reduced its inhibitory effect on sperm-egg binding in an in vitro competition assay, whereas the elimination of O-linked carbohydrate chains by alkali treatment hardly reduced the inhibitory effect. These results indicate that N-linked carbohydrate chains of PZP3 alpha play a major role in mediating the sperm binding of zona pellucida in pig.

Binding of pig sperm receptor in the zona pellucida to the boar sperm acrosome

Pig zona pellucida (ZP) contains three families of glycoproteins: PZP2, PZP3 alpha and PZP3 beta. PZP3 alpha mediates the binding of the ZP to spermatozoa. In this study, the binding site of pig ZP on boar spermatozoa and the zona-binding proteins of boar spermatozoa were studied using chemically modified zona glycoproteins or anti-pig ZP antiserum. Endo-beta-galactosidase-digested PZP3 alpha (E beta G-PZP3 alpha), which is deficient in sulfated N-acetylpolyactosamine, as well as solubilized ZP, bound to the acrosomal region of acrosome-damaged or partially acrosome-reacted spermatozoa. However, they did not bind to acrosome-intact or fully acrosome-reacted spermatozoa. Solubilized ZP did bind to the acrosomal cap released upon acrosome reaction. In western blot analyses, E beta G-PZP3 alpha bound to the sperm proteins with molecular masses similar to proacrosin-acrosin and the binding was inhibited by fucoidan and anti-pig acrosin antiserum. These results suggest that the binding site of solubilized pig ZP and E beta G-PZP3 alpha on spermatozoa is located mainly in the acrosomal matrix and on the membranous compartments in the acrosome and suggest that E beta G-PZP3 alpha binds to proacrosin-acrosin. The binding of E beta G-PZP3 alpha to proacrosin-acrosin may be involved in the binding of the ZP to the acrosome of partially acrosome-reacted spermatozoa.

Characterization of the zona pellucida glycoproteins from bovine ovarian and fertilized eggs

Bovine zona pellucida (ZP) glycoproteins from ovarian egg emerged as three bands with molecular mass of 78 kDa, 64 kDa and 21 kDa in SDS-PAGE under reducing conditions. Endo-beta-galactosidase (E beta G) digestion of the glycoproteins yielded five products with molecular mass of 76 kDa (E beta G-76), 68 kDa (E beta G-68), 63 kDa (E beta G-63), 47 kDa (E beta G-47) and 21 kDa (E beta G-21) under the same conditions. The N-terminal amino acid sequences of E beta G-76 and E beta G-21 were identical. This fact together with the results of diagonal SDS-PAGE indicated that E beta G-21 (N-terminal region) is linked to E beta G-63 (C-terminal region) through disulfide bond to form E beta G-76. Immunoblot analysis using anti-pig ZP protein antibodies revealed that bovine E beta G-76, E beta G-68 and E beta G-47 correspond to pig PZP2, PZP3 alpha and PZP3 beta glycoproteins, respectively. The E beta G-76 and E beta G-68 components were shown to be specifically cleaved during fertilization.

Purification and characterization of a novel acrosin-like enzyme from boar cauda epididymal sperm

A trypsin-like protease was extracted with 1% cetyltrimethylammonium bromide (CTAB) at pH 7.0 from boar cauda epididymal sperm nuclei whose acrosin had previously been removed by acid extraction. The CTAB-extracted sperm protease (CSP) was purified by ion-exchange chromatography on CM-23, gel filtration on Sephadex G-100, affinity chromatography on benzamidine-CH-Sepharose 4B, and HPLC on CM-5PW. CSP is a two chain protein composed of M(r) 2.6K and M(r) 37K chains, which are covalently cross-linked by disulfide bonds. CSP exhibited a pH optimum between pH 8.0 and 9.0, and was inhibited by diisopropyl phosphorofluoridate, antipain, leupeptin, and 1-chloro-3-tosylamide-7-amino-L-2-heptanone. The activity of CSP was enhanced about 1.2-fold with 50 mM CaCl2, with which acrosin is enhanced 2.0-fold. The catalytic efficiency (kcat/Km) of CSP toward Bz-L-Arg-OEt, Tos-L-Arg-OMe, and Tos-L-Lys-OMe in the presence of 50 mM CaCl2 differed from that of acrosin by factors of 0.53, 1.2, and 0.80, respectively. Amino acid sequencing of V8-digested peptides of CSP, and its L- and H-chains showed that the amino acid sequence of CSP was closely related to, but different from, that of acrosin. These results suggest that CSP is a novel acrosin-like enzyme that differs from acrosin in its location in the sperm head, the effect of calcium ions on its activity, and its substrate specificity.

pH-sensitive dissociation and association of beta-N-acetylhexosaminidase from boar sperm acrosome

beta-N-Acetylhexosaminidase (beta-Hex, EC, 3.2.1.52) was released from cauda epididymal boar sperm by treatment with ionophore A23187, indicating that this enzyme is localized in the acrosome. beta-Hex was extracted on a large scale, with 2% acetic acid containing 0.2% Brij 35, from washed ejaculated sperm. By gel filtration chromatography, beta-Hex was separated into a high-molecular-weight fraction (beta-Hex I) and a low-molecular-weight fraction (beta-Hex I). beta-Hex I, which is predominant under acidic conditions (pH 6.5), dissociated into beta-Hex II under alkaline conditions (pH 7.4). beta-Hex II, converted from beta-Hex I, associated again to form beta-Hex I under acidic conditions. By sequential chromatography on ion-exchange, lectin, gel filtration, and ion-exchange HPLC columns, beta-Hex I and II were purified 1200-fold and 4000-fold, respectively, with a combined recovery of 23% as measured with synthetic substrate. An inhibitor of beta-Hex, O-(2-acetamido-2-deoxy-D-glucopyranosylidene) amino N-phenyl-carbamate (PUGNAC), reduced the in vitro fertilization rate in porcine cumulus-enclosed eggs, but barely changed the rate when cumulus-free eggs were used. beta-Hex I was shown to possess cumulus dispersion activity, suggesting that beta-Hex plays a role in the passing by sperm through cumulus cells before they bind to the zona pellucida.

Native zona pellucida structure is required for completion of sperm acrosome reaction in porcine fertilization

In fertilization in vitro, the penetration rate of zona-intact porcine oocytes by cryopreserved epididymal spermatozoa was about 100% while that of zona-free oocytes was only 30%. Spermatozoa treated with calcium ionophore A23187 penetrated both zona-intact and zona-free oocytes at the rate of more than 90%. Treatment of spermatozoa with solubilized procine zonae pellucidae hardly induced acrosome reaction and did not increase the penetration rate. These results suggest that the structure of the zona is necessary for completion of acrosome reaction.

Mutational analysis of an actin-binding site of cofilin and characterization of chimeric proteins between cofilin and destrin

Cofilin and destrin are two related low molecular weight mammalian actin-binding proteins. Cofilin is an F-actin side-binding and pH-dependent actin-depolymerizing protein, and destrin is a pH-independent actin-depolymerizing protein. We have introduced a few point mutations within an actin-binding sequence of cofilin. Biochemical analyses of these mutant proteins have clearly shown that Lys112 and Lys114 of cofilin are crucially but differently involved in its interaction with actin and phosphatidylinositol 4,5-bisphosphate. This is the first example among actin-binding proteins whose point mutations inactivate their interaction with actin in vitro. We have also made and characterized a series of chimeric proteins between cofilin and destrin to identify the regions responsible for the pH dependence and the F-actin side binding activity of cofilin. Our results suggest that a central region consisting of 42 amino acid residues and a carboxyl-terminal quarter of cofilin are both involved in regulation of the pH-dependent actin depolymerizing activity and the activity to bind along F-actin.

A short sequence responsible for both phosphoinositide binding and actin binding activities of cofilin

Cofilin is a widely distributed actin-modulating protein that has abilities to bind along the side of F-actin and to depolymerize F-actin. Both abilities of cofilin can be inhibited by phosphoinositides such as phosphatidylinositol, phosphatidylinositol 4-monophosphate, and phosphatidylinositol 4,5-bisphosphate (PIP2). We have previously shown that the synthetic dodecapeptide corresponding to Trp104-Met115 of cofilin is a potent inhibitor of actin polymerization (Yonezawa, N., Nishida, E., Iida, K., Kumagai, H., Yahara, I., and Sakai, H. (1991) J. Biol. Chem. 266, 10485-10489). In this study, we have found that the inhibitory effect of the synthetic dodecapeptide on actin polymerization is canceled specifically by phosphatidylinositol, phosphatidylinositol 4-monophosphate and PIP2. We further show that the dodecapeptide as well as cofilin binds to PIP2 molecules and inhibits PIP2 hydrolysis by phospholipase C. Thus, the actin-binding dodecapeptide sequence of cofilin may constitute a multifunctional domain in cofilin.

Inhibition of actin polymerization by a synthetic dodecapeptide patterned on the sequence around the actin-binding site of cofilin

Cofilin is an F-actin side-binding and -depolymerizing protein with an apparent molecular mass of 21 kDa. By means of the end label fingerprinting method, the amino acid residue on cofilin sequence cross-linked to actin by zero length cross-linker, 1-ethyl-3-(3-dimethylamino propyl)carbodiimide, was identified as Lys112 and/or Lys114. A synthetic dodecapeptide patterned on the sequence around the actin-cross-linking site of cofilin (Trp104-Met115) inhibited the binding of cofilin to actin. Moreover, the dodecapeptide was found to be a potent inhibitor of actin polymerization. Thus, we conclude that the dodecapeptide sequence constitutes the region essential for the actin-binding and -depolymerizing activity of cofilin. A sequence similar to the dodecapeptide is found in other actin-depolymerizing proteins, destrin, actin-depolymerizing factor, and depactin. Therefore, the dodecapeptide sequence may be a consensus sequence essential for actin-binding and -depolymerizing activity in actin-depolymerizing proteins.

Inhibition of the interactions of cofilin, destrin, and deoxyribonuclease I with actin by phosphoinositides

Cofilin is a widely distributed actin-modulating protein that has the ability to bind along the side of F-actin and to depolymerize F-actin in a pH-dependent manner. We found that phosphatidylinositol (PI), phosphatidylinositol 4-monophosphate (PIP), and phosphatidylinositol 4,5-bisphosphate (PIP2) inhibited both actions of cofilin in a dose-dependent manner, while inositol 1,4,5-triphosphate (IP3), 1-oleoyl-2-acetylglycerol (OAG), phosphatidylserine (PS), or phosphatidylcholine (PC) had little or no effect on them. Gel filtration analyses showed that PIP2 bound to cofilin and thereby inhibited the binding of cofilin to G-actin. Destrin is a mammalian, pH-independent actin-depolymerizing protein. The actin-depolymerizing activity of destrin was also inhibited by PI, PIP, and PIP2, but not by IP3, OAG, PS, or PC. In addition, we found further that an actin-depolymerizing activity of bovine pancreas deoxyribonuclease I, a G-actin-sequestering protein, was inhibited by PIP and PIP2, but not by PI, IP3, OAG, PS, or PC. These results together with previous findings (Lassing, I., and Lindberg, U. (1985) Nature 314, 472-474; Janmey, P. A., and Stossel, T. P. (1987) Nature 325, 362-364) suggest that the sensitivity to polyphosphoinositides may be a common feature in vitro among actin-binding proteins that can bind to G-actin and regulate the state of actin polymerization.

Destrin, a mammalian actin-depolymerizing protein, is closely related to cofilin. Cloning and expression of porcine brain destrin cDNA

Destrin is a mammalian 19-kDa protein that rapidly depolymerizes F-actin in a stoichiometric manner. In this study, we isolated cDNA clones coding for destrin from a porcine brain cDNA library. The deduced amino acid sequence of destrin is 165 residues long and is very similar (71% identical) to that of cofilin, a widely distributed, pH-sensitive actin-modulating protein. Destrin contains a sequence nearly identical with the putative nuclear transport signal sequence of cofilin and a hexapeptide sequence identical with the amino-terminal sequence (residues 2-7) of tropomyosin, which is shown to be involved in cofilin binding to actin. Destrin, like cofilin, also has in its carboxyl-terminal portion a region homologous to the sequence shared by gelsolin, fragmin, and Acanthamoeba profilin. We have expressed destrin as well as cofilin in Escherichia coli, purified them, and examined their function in vitro. The two proteins were found to differ in their interaction with actin, like destrin and cofilin isolated from porcine brain. This suggests that the difference in the function of the two proteins results from the subtle difference in their amino acid sequence rather than possible differences in post-translational modifications. Northern blot analyses indicated that both destrin mRNA and cofilin mRNA are widely distributed in various tissues, but both mRNAs differ in their relative abundance among tissues.

An actin-interacting heptapeptide in the cofilin sequence

Cofilin, a 21-kDa actin-binding protein, has a hexapeptide sequence DAIKKK which is identical to the N-terminal portion (residues 2-7) of tropomyosin. The synthetic heptapeptide, DAIKKKL, corresponding to residues 122-128 of cofilin, inhibited the binding of cofilin to F-actin in a dose-dependent manner. The heptapeptide cosedimented with F-actin, decreased the fluorescence intensity of pyrene-labeled F-actin, and increased the rate of polymerization of G-actin. The hexapeptides, DIKKKL and DAIKKL, also inhibited the binding of cofilin to F-actin and affected the fluorescence intensity of pyrene-labeled F-actin and the rate of actin polymerization, like the heptapeptide. However, their effects were weaker than those of the heptapeptide. Moreover, the pentapeptide, DIKKL, had little or no effect. These results suggest that the heptapeptide sequence is specific for the interaction with actin and, therefore, may constitute part of the actin-binding domain of cofilin.