CiteULike: cactus em

Paired beta-sheet structure of an Abeta(1-40) amyloid fibril revealed by electron microscopy

Carsten Sachse — 2008-06-19T07:56:53-00:00

Proceedings of the National Academy of Sciences, Vol. 105, No. 21. (27 May 2008), pp. 7462-7466.

Alzheimer's disease is a neurodegenerative disorder that is characterized by the cerebral deposition of amyloid fibrils formed by A peptide. Despite their prevalence in Alzheimer's and other neurodegenerative diseases, important details of the structure of amyloid fibrils remain unknown. Here, we present a three-dimensional structure of a mature amyloid fibril formed by A(1-40) peptide, determined by electron cryomicroscopy at approx8-A resolution. The fibril consists of two protofilaments, each containing approx5-nm-long regions of -sheet structure. A local twofold symmetry within each region suggests that pairs of -sheets are formed from equivalent parts of two A(1-40) peptides contained in each protofilament. The pairing occurs via tightly packed interfaces, reminiscent of recently reported steric zipper structures. However, unlike these previous structures, the -sheet pairing is observed within an amyloid fibril and includes significantly longer amino acid sequences. 10.1073/pnas.0712290105

Conservation of the regulated structure of folded myosin 2 in species separated by at least 600 million years of independent evolution

Hyun Jung — 2008-05-07T06:28:28-00:00

Proceedings of the National Academy of Sciences, Vol. 105, No. 16. (22 April 2008), pp. 6022-6026.

The myosin 2 family of molecular motors includes isoforms regulated in different ways. Vertebrate smooth-muscle myosin is activated by phosphorylation of the regulatory light chain, whereas scallop striated adductor-muscle myosin is activated by direct calcium binding to its essential light chain. The paired heads of inhibited molecules from myosins regulated by phosphorylation have an asymmetric arrangement with motor-motor interactions. It was unknown whether such interactions were a common motif for inactivation used in other forms of myosin-linked regulation. Using electron microscopy and single-particle image processing, we show that indistinguishable structures are indeed found in myosins and heavy meromyosins isolated from scallop striated adductor muscle and turkey gizzard smooth muscle. The similarities extend beyond the shapes of the heads and interactions between them: In both myosins, the tail folds into three segments, apparently at identical sites; all three segments are in close association outside the head region; and two segments are associated in the same way with one head in the asymmetric arrangement. Thus, these organisms, which have different regulatory mechanisms and diverged from a common ancestor >600 Myr ago, have the same quaternary structure. Conservation across such a large evolutionary distance suggests that this conformation is of fundamental functional importance. 10.1073/pnas.0707846105

Evidence for an Interaction between the SH3 Domain and the N-terminal Extension of the Essential Light Chain in Class II Myosins

Susan Lowey — 2008-04-17T08:30:26-00:00

Journal of Molecular Biology, Vol. 371, No. 4. (24 August 2007), pp. 902-913.

The function of the src-homology 3 (SH3) domain in class II myosins, a distinct [beta]-barrel structure, remains unknown. Here, we provide evidence, using electron cryomicroscopy, in conjunction with light-scattering, fluorescence and kinetic analyses, that the SH3 domain facilitates the binding of the N-terminal extension of the essential light chain isoform (ELC-1) to actin. The 41 residue extension contains four conserved lysine residues followed by a repeating sequence of seven Pro/Ala residues. It is widely believed that the highly charged region interacts with actin, while the Pro/Ala-rich sequence forms a rigid tether that bridges the ~ 9 nm distance between the myosin lever arm and the thin filament. In order to localize the N terminus of ELC in the actomyosin complex, an engineered Cys was reacted with undecagold-maleimide, and the labeled ELC was exchanged into myosin subfragment-1 (S1). Electron cryomicroscopy of S1-bound actin filaments, together with computer-based docking of the skeletal S1 crystal structure into 3D reconstructions, showed a well-defined peak for the gold cluster near the SH3 domain. Given that SH3 domains are known to bind proline-rich ligands, we suggest that the N-terminal extension of ELC interacts with actin and modulates myosin kinetics by binding to the SH3 domain during the ATPase cycle.

Large conformational changes in a kinesin motor catalyzed by interaction with microtubules.

K Hirose — 2007-07-27T06:57:25-00:00

Mol Cell, Vol. 23, No. 6. (15 September 2006), pp. 913-923.

Kinesin motor proteins release nucleotide upon interaction with microtubules (MTs), then bind and hydrolyze ATP to move along the MT. Although crystal structures of kinesin motors bound to nucleotides have been solved, nucleotide-free structures have not. Here, using cryomicroscopy and three-dimensional (3D) reconstruction, we report the structure of MTs decorated with a Kinesin-14 motor, Kar3, in the nucleotide-free state, as well as with ADP and AMPPNP, with resolution sufficient to show alpha helices. We find large structural changes in the empty motor, including melting of the switch II helix alpha4, closure of the nucleotide binding pocket, and changes in the central beta sheet reminiscent of those reported for nucleotide-free myosin crystal structures. We propose that the switch II region of the motor controls docking of the Kar3 neck by conformational changes in the central beta sheet, similar to myosin, rather than by rotation of the motor domain, as proposed for the Kif1A kinesin motor.

Three-dimensional structure of vertebrate cardiac muscle myosin filaments

Maria Zoghbi — 2008-02-22T09:20:05-00:00

Proceedings of the National Academy of Sciences, Vol. 105, No. 7. (19 February 2008), pp. 2386-2390.

Contraction of the heart results from interaction of the myosin and actin filaments. Cardiac myosin filaments consist of the molecular motor myosin II, the sarcomeric template protein, titin, and the cardiac modulatory protein, myosin binding protein C (MyBP-C). Inherited hypertrophic cardiomyopathy (HCM) is a disease caused mainly by mutations in these proteins. The structure of cardiac myosin filaments and the alterations caused by HCM mutations are unknown. We have used electron microscopy and image analysis to determine the three-dimensional structure of myosin filaments from wild-type mouse cardiac muscle and from a MyBP-C knockout model for HCM. Three-dimensional reconstruction of the wild-type filament reveals the conformation of the myosin heads and the organization of titin and MyBP-C at 4 nm resolution. Myosin heads appear to interact with each other intramolecularly, as in off-state smooth muscle myosin [Wendt T, Taylor D, Trybus KM, Taylor K (2001) Proc Natl Acad Sci USA 98:43614366], suggesting that all relaxed muscle myosin IIs may adopt this conformation. Titin domains run in an elongated strand along the filament surface, where they appear to interact with part of MyBP-C and with the myosin backbone. In the knockout filament, some of the myosin head interactions are disrupted, suggesting that MyBP-C is important for normal relaxation of the filament. These observations provide key insights into the role of the myosin filament in cardiac contraction, assembly, and disease. The techniques we have developed should be useful in studying the structural basis of other myosin-related HCM diseases. 10.1073/pnas.0708912105