The giant sarcomeric protein titin contains a protein kinase domain (TK) ideally positioned to sense mechanical load. We identified a signaling complex where TK interacts with the zinc-finger protein nbr1 through a mechanically inducible conformation. Nbr1 targets the ubiquitin-associated p62/SQSTM1 to sarcomeres, and p62 in turn interacts with MuRF2, a muscle-specific RING-B-box E3 ligase and ligand of the transactivation domain of the serum response transcription factor (SRF). Nuclear translocation of MuRF2 was induced by mechanical inactivity and caused reduction of nuclear SRF and repression of transcription. A human mutation in the titin protein kinase domain causes hereditary muscle disease by disrupting this pathway.During muscle differentiation, a specific program of gene expression leads to the translation of myofibrillar proteins and their assembly into contractile units, the sarcomeres, which are constantly remodeled to adapt to changes in mechanical load. The giant protein titin (also known as connectin) acts as a molecular blueprint for sarcomere assembly by providing specific attachment sites for numerous sarcomeric proteins, as well as acting as a molecular spring (1, 2). Titin also contains a catalytic serine-threonine kinase domain (TK), which is inhibited by a specific dual mechanism (3). However, the upstream elements controlling TK activation, its range of cellular substrates, and particularly the role of TK in mature muscle are largely unknown. Spanning half sarcomeres from Z disk to M band, titin is in a unique position to sense mechanical strain along the sarcomere (1). The elastic properties of the titin molecule and the mechanical deformation of the M band during stretch and contraction (4) suggest that the signaling properties of TK might be modulated by mechanically induced conformational changes. Molecular dynamics simulations suggest that mechanical strain can induce a catalytically active conformation of TK (5).The catalytic kinase domain of titin interacts with nbr1. We searched for further elements of a putative signaling pathway that might recognize mechanically induced conformational intermediates of titin's catalytic domain. In a systematic two-hybrid screening approach with various structure-based open states of the catalytic site [kin1, kin2, and kin3 (6)], we identified the zinc-finger protein nbr1 (7) as a TK ligand, which interacted via its Nterminal PB1 domain with the semiopened construct kin3 (Fig. 1, A and B). This interaction was also seen in precipitation experiments with nbr1 and TK-kin3 ( fig. S1A). Kin1, where the complete regulatory domain closes the active site, and kin2, where the a helix R1 (3) is deleted, did not interact. Thus, aR1 was necessary but not sufficient for nbr1 binding, which also required a semiopened catalyt-

Precise apposition of pre- to postsynaptic specializations is required for optimal function of chemical synapses, but little is known about how it is achieved. At the skeletal neuromuscular junction, active zones (transmitter release sites) in the nerve terminal lie directly opposite junctional folds in the postsynaptic membrane. Few active zones or junctional folds form in mice lacking the laminin beta2 chain, which is normally concentrated in the synaptic cleft. beta2 and the broadly expressed gamma1 chain form heterotrimers with alpha chains, three of which, alpha2, alpha4 and alpha5, are present in the synaptic cleft. Thus, alpha2beta2gamma1, alpha4beta2gamma1 and alpha5beta2gamma1 heterotrimers are all lost in beta2 mutants. In mice lacking laminin alpha4, active zones and junctional folds form in normal numbers, but are not precisely apposed to each other. Thus, formation and localization of synaptic specializations are regulated separately, and alpha4beta2gamma1 (called laminin-9) is critical in the latter process.

Acute quadriplegic myopathy is associated with a specific decrease in thick-filament proteins related to an altered transcription rate. Although the decreased content of thick-filament proteins is important for prolonged muscle weakness, it is not the primary cause of muscle paralysis in the acute stage, during which impaired muscle membrane excitability probably plays a more significant role. Several factors contribute to this condition, but the action of corticosteroids seems to be the predominant one, along with potentiation by neuromuscular blocking agents, immobilization, and probably also concurrent sepsis.