Conformational change of skeletal muscle alpha-actinin induced by salt

Limited digestion of α-actinin in the presence of F-actin

N-terminal actin-binding domain of α-actinin is connected to central rod domain through flexible neck region that is susceptible to proteolysis. It is suggested that the neck region assumes variable orientations by actin binding. In order to examine the effect of actin binding to α-actinin, we carried out limited digestion of α-actinin by chymotrypsin in the presence and absence of F-actin. Although the cleavage process was retarded when bound to F-actin, digestion to 32 kDa-head and 55 kDa-rod domains occurred through the same intermediate products as the digestion in the absence of F-actin. N-terminal sequencing of 55 kDa-fragment showed the neck region was cleaved at 276-Leu. The cleavage site was not affected by binding to F-actin nor ionic strength of the solvent. It was also indicated that α-actinin was cleaved at 15-Tyr by chymotrypsin. Quantitation of the cleavage products by densitometry of the SDS-gels suggested the conformational change of α-actinin at domain-connecting regions by F-actin binding.

The regulatory action of alpha-actinin on actin filaments is enhanced by cofilin

We have used fluorescence recovery after photobleaching to study the effect of muscle alpha-actinin on the structure of actin filaments in dilute solutions. Unexpectedly we found that alpha-actinin partitioned filaments into two types: those with a high mobility and those with low mobility. We have determined that the high mobility (smaller sized) population is too large to be simple monomeric actin:alpha-actinin complexes. Although it is known that cofilin encourages the transformation of alpha-actinin:actin gels into large meshworks of inter-digitating actin filament bundles (Maciver et al. 1991), we have found that the presence of cofilin also increases the cross-linking of actin filaments by alpha-actinin and hypothesize that this is due to cofilin's ability to alter the filament twist. This effectively makes more potential alpha-actinin binding sites per unit of actin filament. As expected from previous work, this effect was more marked at pH 6.5 than at pH 8.0. Both effects are likely to operate in cells to deny other actin-binding proteins access to binding these particular filaments and may explain how very different actin cytoskeletal structures may co-exist in the same cell at the same time.

Protein-lipid interactions: correlation of a predictive algorithm for lipid-binding sites with three-dimensional structural data

Background

Over the past decade our laboratory has focused on understanding how soluble cytoskeleton-associated proteins interact with membranes and other lipid aggregates. Many protein domains mediating specific cell membrane interactions appear by fluorescence microscopy and other precision techniques to be partially inserted into the lipid bilayer. It is unclear whether these protein-lipid-interactions are dependent on shared protein motifs or unique regional physiochemistry, or are due to more global characteristics of the protein.

Results

We have developed a novel computational program that predicts a protein's lipid-binding site(s) from primary sequence data. Hydrophobic labeling, Fourier transform infrared spectroscopy (FTIR), film balance, T-jump, CD spectroscopy and calorimetry experiments confirm that the interfaces predicted for several key cytoskeletal proteins (alpha-actinin, Arp2, CapZ, talin and vinculin) partially insert into lipid aggregates. The validity of these predictions is supported by an analysis of the available three-dimensional structural data. The lipid interfaces predicted by our algorithm generally contain energetically favorable secondary structures (e.g., an amphipathic alpha-helix flanked by a flexible hinge or loop region), are solvent-exposed in the intact protein, and possess favorable local or global electrostatic properties.

Conclusion

At present, there are few reliable methods to determine the region of a protein that mediates biologically important interactions with lipids or lipid aggregates. Our matrix-based algorithm predicts lipid interaction sites that are consistent with the available biochemical and structural data. To determine whether these sites are indeed correctly identified, and whether use of the algorithm can be safely extended to other classes of proteins, will require further mapping of these sites, including genetic manipulation and/or targeted crystallography.

Smooth muscle alpha-actinin binds tightly to fesselin and attenuates its activity toward actin polymerization

Fesselin is an actin binding protein from smooth muscle that nucleates actin polymerization in a Ca(++)-calmodulin dependent manner, bundles actin and inhibits the actin-activated ATPase activity of myosin S1. We now report that fesselin binds to smooth muscle alpha-actinin. Binding was measured by blot overlay, affinity chromatography and sedimentation methods. Binding was moderate with an association constant of 1-4 x 10(7) M(-1) assuming a 1:1 association of fesselin with alpha-actinin. Fesselin binds to the central spectrin domain repeat region of alpha-actinin but not to the CH1-CH2 domain. Fesselin accelerates the polymerization of actin. This activity of fesselin was attenuated by alpha-actinin. These observations support the role of fesselin in organizing the cytoskeleton.

Actomyosin interactions in a novel single muscle fiber in vitro motility assay

A novel in vitro motility assay has been developed to study the actomyosin interaction, in which the molecular motor protein myosin has been extracted and immobilized directly from 2-4 mm single rat skeletal muscle fiber segments. This method study was carried out to investigate: (1) the amount of myofibrillar proteins extracted from the fiber segment; (2) the effects of temperature on the speed at which actin is propelled by fast and slow myosin; and (3) the effects of myosin isoform expression on motility speed. Approximately 80% of the myosin and myosin-associated proteins were extracted from the fiber segments. while no significant extraction was shown of the thin filament proteins. Fluorescently labeled actin filaments moved with constant speed in a bi-directional motion over the high-density myosin region in the experimental chamber, and motility speed was highly dependent on the myosin heavy chain (MyHC) isoform extracted. At 25 degrees C, significant (P < 0.001) differences in motility speed were obtained between type I (1.31 +/- 0.23 microm/s, n = 11) and IIxb (5.81 +/- 0.35 microm/s, n = 6), or llb (6.07 +/- 0.33 microm/s, n = 8) MyHC isoforms. The motility speed and maximum velocity of unloaded shortening (V0) in single fibers were well correlated, indicating that filament speed is a good molecular analogue to contractile speed at the fiber level. The effects of temperature on filament motility speed were analyzed from 10 to 35 degrees C. The Q10 values, calculated in the 10-25 degrees C temperature range, differed between slow (4.20) and fast (2.38) myosin. In conclusion, this in vitro motility assay offers a unique possibility to compare the regulatory and modulatory influence of myosin isoforms and thin filament proteins on shortening velocity, at the cellular and molecular level in the same muscle fiber.

Structure of the alpha-actinin rod: molecular basis for cross-linking of actin filaments

We have determined the crystal structure of the two central repeats in the alpha-actinin rod at 2.5 A resolution. The repeats are connected by a helical linker and form a symmetric, antiparallel dimer in which the repeats are aligned rather than staggered. Using this structure, which reveals the structural principle that governs the architecture of alpha-actinin, we have devised a plausible model of the entire alpha-actinin rod. The electrostatic properties explain how the two alpha-actinin subunits assemble in an antiparallel fashion, placing the actin-binding sites at both ends of the rod. This molecular architecture results in a protein that is able to form cross-links between actin filaments.

FTIR study of the thermal denaturation of alpha-actinin in its lipid-free and dioleoylphosphatidylglycerol-bound states and the central and N-terminal domains of alpha-actinin in D2O

Fourier transform infrared (FTIR) spectroscopy has been carried out to investigate the thermal denaturation of alpha-actinin and its complexes with dioleoylphosphatidylglycerol (DOPG) vesicles. The amide I regions in the deconvolved spectra of alpha-actinin in the lipid-free and DOPG-bound states are both consistent with predominantly alpha-helical secondary structure below the denaturation temperatures. Studies of the temperature dependence of the spectra revealed that for alpha-actinin alone the secondary structure was unaltered up to 40 degrees C. But, in the presence of DOPG vesicles, the thermal stability of the secondary structure of alpha-actinin increased to 55 degrees C. The thermal denaturation mechanisms of the lipid-free and DOPG-bound states of alpha-actinin also vary. The secondary structure of the lipid-free alpha-actinin changed to be predominantly unordered upon heating to 65 degrees C and above. Whereas, the original alpha-helical structure in the DOPG-bound alpha-actinin retained even at 70 degrees C, the highest temperature we examined. Analysis of the reduction in amide II intensities, which is due to peptide H-D exchange upon heating alpha-actinin in D2O, showed that partially unfolded states with increased solvent accessibility but substantial secondary structures could be observed from 35 to 40 degrees C only if DOPG vesicles were present. A so-called "protamine precipitation" method has been developed to purify the N-terminal domain of alpha-actinin by use of the fact that the central domain of alpha-actinin is negatively charged but the N-terminal domain is positively charged. Thermal denaturation of the central and N-terminal domains of alpha-actinin were then investigated with FTIR. The secondary structure of the N-terminal domain of alpha-actinin was found to be thermally sensitive below 35 degrees C, which is characterized as the increase of the alpha-helical structure at the expense of the random coil upon heating the N-terminal domain from 4 to 35 degrees C. The membrane-binding ability of the N-terminal domain of alpha-actinin was proposed in terms of the analysis of the local electrostatic properties of alpha-actinin and the assignment of the amide II bands in the FTIR spctra of alpha-actinin.

Interactions between smooth muscle alpha-actinin and lipid bilayers

alpha-Actinin has been proposed to be the actin-plasma membrane linker. This assumption is based on the discovery of direct interaction of alpha-actinin with two specific lipids, diacylglycerol and palmitic acid [Burn, P. (1988) Trends Biochem. Sci. 13, 79-83]. In our study, the binding of alpha-actinin with vesicles containing negatively charged phospholipids was measured by the method of 90 degrees light-scattering. Our results show that alpha-actinin is able to bind membranes containing negatively charged phospholipids, but not to bind membranes composed of neutral lipids only. Diacylglycerol and palmitic acid, on the other hand, have little effect on the binding of alpha-actinin to lipid vesicles. Analysis of binding isotherms in terms of a membrane binding model gave apparent dissociation constants which varied between 0.2 and 3 microM over a range of 5-20 mol % negatively charged phospholipid. Comparing the kinetics of alpha-chymotrypsin digestion of alpha-actinin in solution to those of vesicle-bound alpha-actinin, it can be seen that the cleavage site at the junction between the C-terminal and the central rod domain of alpha-actinin and another cleavage site on the C-terminal domain can be most effectively protected by its membrane binding. Analysis of the amide I and II regions of Fourier-transform infrared spectra of alpha-actinin revealed that the association of alpha-actinin with negatively charged phospholipid vesicles resulted in some perturbation of the protein secondary structure. Monolayers containing negatively charged phospholipid were layered and incubated on the surface of a polymerization solution of actin and alpha-actinin, and observed with an electron microscope. The results show that the bundle structure of actin filaments can be formed if diacylglycerol and palmitic acid are present in lipid layers.

Flexibility and fine structure of smooth-muscle alpha-actinin

The microfilament protein alpha-actinin exists as a dimer. The N-terminal regions of both polypeptides, arranged in antiparallel orientation, comprise the actin-binding regions, while the C-terminal, larger parts consist of four spectrin-like repeats that interact to form a rod-like structure. To elucidate the fine structure of smooth-muscle alpha-actinin, we used energy-filtered transmission electron microscopy in conjunction with negative staining. Survey pictures of the protein purified from chicken gizzard revealed discrete, elongated particles whose length and width varied with the ionic strength of the buffer. It was determined to to 29.3 nm x 4.8 nm in 0.05 M KCl and 32.6 nm x 4.4 nm in 0.15 M KCl. Both ends of the molecule displayed hook-like structures consisting of globular domains, which were highly variable in their orientation with respect to the long axis of the molecule. Their location at the ends of the molecule, and the finding that these hooks were missing from particles obtained by thermolysin digestion indicated that they probably correspond to the N-terminal actin-binding regions. The rod-like center of the molecule revealed discrete globular masses which probably comprise the spectrin-like repeats. Their arrangement was compatible with the interpretation that three spectrin repeats of each polypeptide chain can form pairs with the respective sequences of the other chain. The rod-like 53-kDa fragment obtained after thermolysin digestion largely retained this structural organization but appeared wider (22.5 nm x 5.9 nm). Our results help to clarify previous discrepancies on the quatenary organization of alpha-actinin and suggest that effective actin-binding and cross-linking of alpha-actinin is based on the high flexibility of the terminal hooks.

Cation effects on the conformations of muscle and non-muscle alpha-actinins

We examined the effects of changing KCl concentration on the secondary structures of alpha-actinins using circular dichroism (CD), 1,1'-bis(4-anilino) naphthalene-5,5'-disulfonic acid (bisANS) fluorescence and proteolysis experiments. Under near-physiological conditions, divalent cations also were added and changes in conformation were investigated. In 25 mM KH2PO4, pH 7.5, increasing KCl from 0 to 120 mM led to decreases in alpha-helix conformation for brain, platelet and heart alpha-actinins (40.5-30.2%, 65.5-37.8% and 37.5-27.8%, respectively). In buffered 120 mM KCl, 0.65 mM calcium produced small changes in the CD spectra of both brain and platelet alpha-actinin, but had no effect on heart alpha-actinin. bisANS fluorescence of all three alpha-actinins also showed significant changes in conformation with increasing KCl. However, in buffered 120 mM KCl increasing concentrations of Ca2+ or Mg2+ did not have significant effects on the bisANS fluorescence of any alpha-actinin. Digestion of brain, platelet and heart alpha-actinins with alpha-chymotrypsin showed an increase of proteolytic susceptibility in 120 mM KCl. These experiments also showed that increasing the concentration of Ca2+ or Mg2+ led to greater changes in digestion fragment patterns in the absence of KCl than in the presence of 120 mM KCl. The results suggest that alpha-actinins exist in different conformations depending on the ionic strength of the medium, which could explain the differences in calcium and F-actin binding results obtained from different alpha-actinins.

Analysis of the phasing of four spectrin-like repeats in alpha-actinin

Selected fragments of the central rod of chicken gizzard alpha-actinin were expressed as fusion proteins in Escherichia coli, with the aim of determining the positions in the sequence of the four successive spectrin-like repeats that make up this domain. The criteria for an independently folding unit were resistance to proteolysis and the high alpha helicity characteristic of the native protein. Sequences containing repeats 1-4, 2-4, 3-4 and 4 all generated stable fragments on digestion with trypsin and/or thermolysin and N-terminal sequencing gave the most probable starting position of each repeat. The sequences of all four inferred repeats and the sequences of the entire rod, were separately expressed and were shown to assume a stable, protease-resistant fold in solution. The repeat boundaries established in this way differed from those originally deduced from sequence alignments; the N-terminal boundaries of the repeats were 14-24 residues nearer the C-terminus than predicted. The ability to express individual repeats should facilitate identification of the binding sites for the cytoplasmic domains of beta 1 integrins and intercellular cell adhesion molecule-1 which have been localised to the rod domain of alpha-actinin.