Clathrin’s muscle-building regimen

C lathrin is best known for its role in endocytosis and other membrane traffi cking events, but the protein's heavy chain has several additional functions , including organization of mitotic spindles and coordination of actin fi laments at intercellular adhesions. Vassilopoulos et al. now reveal that the clathrin heavy chain (CHC) also helps to organize skeletal muscle sarcomeres by promoting their attachment to muscle cell membranes (1). Stéphane Vassilopoulos, a member of Marc Bitoun's team at the Institut de Myol-ogie in Paris, was interested in CHC's function in skeletal muscle because the protein is expressed in a characteristic, striated pattern in muscle fi bers, overlapping with the actin-rich I-band of the sarcomeres. Clathrin has been proposed to promote myofi bril assembly (2), and mutations in the GTPase dynamin 2, clathrin's partner in both endo-cytosis and actin fi lament organization, cause an autosomal-dominant form of cen-tronuclear myopathy (3, 4). " We wanted to revisit the role of clathrin in skeletal muscle, " Vassilopoulos says. Vassilopoulos et al. examined CHC's distribution in adult muscle fi bers by confocal and electron microscopy (1) and discovered that the protein colocalized with the actin-binding protein ␣-actinin in costameres, peripheral structures where sarcomeres attach to the plasma membrane and surrounding ex-tracellular matrix. Costameres are said to be the " Achilles' heel " of skeletal muscle because the contractile apparatus falls apart when its connection to the plasma membrane is disrupted (5). Vassilopoulos and colleagues then knocked down CHC in cultured muscle cells. ␣-Actinin usually concentrates at the plasma membrane of differentiating muscle cells before adopting a striated pattern of localization to the costameres and sarcomeric Z-discs. In the absence of CHC, however, ␣-actinin ag-gregated in the cytoplasm. " And if you let these cells differentiate for longer, they never become striated, " Vassilopoulos explains. Depleting CHC from the muscle fi bers of adult mice also disrupted the striated pattern of ␣-actinin and F-actin, leading to a loss of contractile properties and muscle degeneration. " There's a strong disorga-nization of the contractile apparatus and large regions of detachment from the plasma membrane, " Vassilopoulos says. To investigate how CHC might promote the attachment and assembly of muscle sar-comeres, Vassilopoulos worked with John Heuser and Robyn Roth at Washington University in St. Louis to observe clath-rin's organization in cultured muscle cells by quick-freeze, deep-etch electron microscopy. This approach revealed that clathrin forms numerous " plaques " on the …