Department of Physiology, University of Pennsylvania School of Medicine
Department of Physiology
771 Clinical Research Building
Philadelphia, PA 19104-6085
Phone: (215) 898-0045
Fax: (215) 573-5851
Position: Senior Research Investigator, Assistant Professor
My research interests have largely focused on mechanisms of contactility and the form and function of the cytoskeleton. Structure and structural change are fundamental aspects of these problems, and I have devoted much time and effort to devising new tools for investigating the structure of macromolecular assemblies.
The structural changes in enzymes that accompany substrate binding, reaction, and product release are generally on a scale of 0.1-0.2 nm; in muscle, adenosine triphosphate (ATP) hydrolysis induces structural changes on a scale of 3-5 nm per crossbridge cycle. Detailed structural information is clearly necessary to understand the mechanisms by which structural change at the crossbridge is produced and regulated.
To obtain more detailed structural information, I devised a method for applying heavy metal labels to chemically defined sites in actin and myosin, allowing them to be located in the 3-dimensional structure by electron microscopy and image analysis. At this point, reliable biochemical procedures have been devised for labeling sites on both actin and myosin subfragment-1 with maleimide-undecagold clusters, and the identification of labeled residues is under way. The location of undecagold bound to cysteine-374 of F-actin has now been determined by low dose cryo-electron microscopy and computational image analysis, in collaboration with Dr. Ron Milligan (Department of Molecular Biology, Research Institute of the Scripps Clinic). Additional sites on actin and on the myosin head will be identified by the same methods. To investigate changes in crossbridge geometry accompanying the hydrolytic cycle, it will be necessary to use gold clusters containing larger numbers of heavy atoms, so that they can be detected in less orderly specimens. Initial experiments on the preparation of such clusters have given encouraging results, and should provide a useful tool for the study of many biological structures.
Working in collaboration with Dr. Vivianne Nachmias, I have discovered and partially characterized a new form of sequestered actin and a new, low molecular weight actin-sequestering protein in platelets. Amino acid sequencing, performed in collaboration with Dr. Marshall Elzinga (Laboratory of Neurobiochemistry, Institute for Basic Research, Staten Island, NY), has shown that this protein is, in fact, thymosin Beta 4. In the coming year we will isolate and further characterize the native complex, study the mechanism of association in vitro (collaboration with Dr. Elzinga), and attempt to grow crystals of the complex suitable for structural studies by X-ray diffraction. Comparison with the structures of actin-DNase, actin-profilin, and F-actin should provide information about the mechanism (steric, conformational, or both) by which actin-sequestering proteins regulate actin polymerization.
- Safer, D., Elzinga, M., and Nachmias, V.T. 1991. Thymosin b4 and Fx, and actin-sequestering peptide, are indistinguishable. J. Biol. Chem. 266:4029-4032.
- Safer, D., Golla, R., and Nachmias, V.T. 1990. Isolation of a 5-kilodalton actin-sequestering peptide from human blood platelets. Proc. Natl. Acad. Sci. USA. 87:2536-2540.
- Milligan, R.A., Whittaker, M., and Safer, D. 1990. Molecular structure of F-actin and location of surface binding sites. Nature. 348:217-221.