Fig. 1.

Actin in vitro and in cyto. a The dynamics of actin filament assembly and disassembly in vitro are regulated by the kind of nucleotide that is bound and actin’s own nuclease activity. Therefore, a fast-growing end (plus end, barbed end) can be defined, where ATP-actin monomers associate and a slow growing end (minus end, pointed end) from which ADP-actin monomers dissociate. Barbed and pointed end refer to the decoration of actin filaments with S1 fragments of myosin (i.e. the myosin heads) in electron micrographs and the resulting arrow-like structures. b Cross-striated muscle cells share the organization of their contractile elements to para-crystalline myofibrils (represented by striations, which amount to 1 µm), but show different organisation at tissue level. Skeletal muscle is composed of multinucleated myofibres that can reach a length of a few millimetres/centimetres depending on the muscle size (only a central part of a skeletal myocyte is shown; indicated by the jagged ends), while cardiac tissue is composed of mono- or binucleated cells with a length of only a few hundred micrometres. c Schematic drawing of part of a myocyte (“hybrid” with characteristics of cardiac and skeletal muscle): the majority of the actin filaments (shown in red) are arranged to thin filaments in the sarcomeres, the basic contractile unit of the myofibril, where they interact with the myosin heads (purple) from the bipolar thick filaments (only one myosin head shown for reasons of simplicity). Sarcomeres are delineated by the Z-discs (alpha-actinin in turquoise). On the left the myofibrils are anchored in the adherens junctions (pink rectangles) at the intercalated disc (assuming the cardiomyocyte scenario), potentially mediated by actin filaments composed of cytoskeletal actins; the last Z-disc before the intercalated disc lacks proteins such as telethonin and is termed the transitional junction (Bennett et al. 2006). In addition, cytoskeletal actins (beta- and gamma-actin) appear to be involved in membrane anchorage around the costameres (green rectangles), together with dystrophin and the dystrophin associated protein complex (dark blue rectangle) and with different parts of the sarcoplasmic reticulum (light blue); T tubules in vertebrate heart muscle are situated above the Z-disc, in skeletal muscle they lie above the A/I-junction. Sarcomeric actin filaments: CapZ in green, nebulin in orange (stretching throughout the thin filament like in skeletal muscle sarcomeres), tropomyosin in grey, the troponin complex in yellow, tropomodulin in blue. A-band: titin in brown, myosin in purple, MyBP-C in turquoise. M-band: myomesin in green, M-protein in brown