When purified muscle actin was mixed with microtubule-associated proteins (MAPs) prepared from brain microtubules assembled in vitro, actin filaments were organized into discrete bundles, 26 nm in diameter . MAP-2 was the principal protein necessary for the formation of the bundles. Analysis of MAP-actin bundle formation by sedimentation and electrophoresis revealed the bundles to be composed of^'20% MAP-2 and 80% actin by weight . Transverse striations were observed to occur at 28-nm intervals along negatively stained MAPactin bundles, and short projections,^'12 nm long and spaced at 28-nm intervals, were resolved by high-resolution metal shadowing. The formation of MAP-actin bundles was inhibited by millimolar concentrations of ATP, AMP-PCP (,ß,y-methylene-adenosine triphosphate), and pyrophosphate but not by AMP, ADP, or GTP. The addition of ATP to a solution containing MAP-actin bundles resulted in the dissociation of the bundles into individual actin filaments; discrete particles, presumably MAP-2, were periodically attached along the splayed filaments. These results demonstrate that MAPS can bind to actin filaments and can induce the reversible formation of actin filament bundles in vitro.During the past decade, increasing evidence has indicated that there is a cytoskeleton within the cytoplasm ofmost eucaryotic cells and that the movements of cells and of organelles within cells occur in association with the cytoskeleton . Although the protein composition ofthe cytoskeleton is not well understood, the best evidence suggests that the major cytoskeletal structures include actin filaments, microtubules, and "intermediate filaments" (22,27). If cytoplasmic movements occur in association with the cytoskeletal structures, then it is likely that the structures interact with one another to mediate the movements. For example, the apparent presence of actin, microtubules, and myosin in the mitotic apparatus (6,12,13,28,33) suggests that these structures may interact with one another to move the chromosomes. Axoplasmic transport may also require associations between microtubules, actin, intermediate filaments, or membrane vesicles (1,3,20,23,35) .Although it is tempting to speculate that associations between cytoskeletal structures are responsible for cell movements, there is little direct biochemical evidence to illustrate how these interactions may occur . Griffith and Pollard (15) recently presented viscometric and ultrastructural evidence for the interaction of actin filaments with microtubules, in addition to viscometric evidence of the interaction of actin with micro-THE JOURNAL OF CELL BIOLOGY " VOLUME 90 AUGUST 1981 467-473 ©The Rockefeller University Press " 0021-9525/81/08/0467/07 $1 .00 tubule-associated proteins (MAPs). Moreover, the associations were reversible upon the addition ofATP, ITP, pyrophosphate, and, to a lesser extent, other nucleotides .In this report, we present evidence that MAPs, in particular MAP-2, can reversibly induce the assembly of actin into discrete bundles offilaments co...

The interaction of unphosphorylated and phosphorylated microtubule-associated protein 2 (MAP-2) with actin filaments was examined by electron microscopic, electrophoretic, and dark-field light microscopic techniques. Unphosphorylated MAP-2 was observed to cross-link and bundle individual actin filaments. Chymotryptic fragments of MAP-2 protein were produced which bound to, but could not cross-link, actin polymer; these fragments encompassed the tubulin binding domain of MAP-2. The phosphorylation of intact MAP-2, by means of endogenous protein kinases, inhibited the ability of this molecule to cross-link and bundle actin filaments. Phosphorylation did not, however, inhibit the binding of MAP-2 to F-actin. The chymotryptic fragments of phosphorylated MAP-2 that retained their ability to bind to actin and promote microtubule assembly also encompassed the tubulin binding domain of this molecule. An analysis of MAP-2 fragments by nonequilibrium pH gradient electrophoresis indicated that most of the polypeptide backbone is relatively acidic with the exception of the tubulin binding domain. This region was determined to be the most basic (positively charged) region of the MAP-2 molecule. Biochemical and morphological evidence is presented to demonstrate that both unphosphorylated MAP-2 and phosphorylated MAP-2 have the capacity to use actin, in addition to microtubules, as a separate anchoring substrate. The presence of tubulin, however, strongly inhibits the interaction of MAP-2 with actin filaments.

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inorganic ions are suggested as mildly-acting dispersal solutions on the basis of evaluations by phase contrast microscopy, Papanicolaou staining and particle volume analysis. Cells are held together by junctional complexes (7, 14, 17, 22) and surface glycoproteins (18, 20, 26). The proteolytic enzymes, chelating agents, detergents and mechanical forces usually employed for cell dispersal act on these cell constituents only because they act on all cell constituents. This study explores the specific chemical features of junctional complexes and the cell gylcocahyx as they apply to the dispersal of cells from gynecologic specimens.

Atomic absorption spectroscopy, electron microprobe analysis, and dithizone staining of trophozoites and cysts of Entamoeba invadens demonstrate that these cells have a high concentration of zinc (approximately 10 -6 microgram per cell or 1 percent of their dry weight). In the cysts of this organism, the zinc is confined to the chromatoid bodies, which previous work has shown to contain crystals of ribosomes. The chemical state and function of this zinc are unknown.