Scaffolds and chaperones in myofibril assembly: putting the striations in striated muscle

Abstract

Sarcomere assembly in striated muscles has long been described as a series of steps leading to assembly of individual proteins into thick filaments, thin filaments and Z-lines. Decades of previous work focused on the order in which various structural proteins adopted the striated organization typical of mature myofibrils. These studies led to the view that actin and α-actinin assemble into premyofibril structures separately from myosin filaments, and that these structures are then assembled into myofibrils with centered myosin filaments and actin filaments anchored at the Z-lines. More recent studies have shown that particular scaffolding proteins and chaperone proteins are required for individual steps in assembly. Here, we review the evidence that N-RAP, a LIM domain and nebulin repeat protein, scaffolds assembly of actin and α-actinin into I-Z-I structures in the first steps of assembly; that the heat shock chaperone proteins Hsp90 & Hsc70 cooperate with UNC-45 to direct the folding of muscle myosin and its assembly into thick filaments; and that the kelch repeat protein Krp1 promotes lateral fusion of premyofibril structures to form mature striated myofibrils. The evidence shows that myofibril assembly is a complex process that requires the action of particular catalysts and scaffolds at individual steps. The scaffolds and chaperones required for assembly are potential regulators of myofibrillogenesis, and abnormal function of these proteins caused by mutation or pathological processes could in principle contribute to diseases of cardiac and skeletal muscles.

Fig. 1

A putative pathway for myofibril assembly highlighting the role of transiently associated proteins in organizing the major structural components (modified from Greenberg et al. 2008). (1) N-RAP promotes assembly of the I-Z-I structures containing actin, α-actinin, and N-terminal titin. (2) Myosin filaments form separately, with appropriate folding and assembly promoted by the Hsc70 and Hsp90 chaperone proteins and the UNC-45 co-chaperone. (3) Titin associates with the myosin filaments along their length, helping to integrate the thick filaments with the I-Z-I structures. In addition, myomesin interacts with titin and myosin to crosslink the array of myosin filaments at the center of the sarcomere. This gives rise to thin myofibrils. (4) Finally, Krp1 promotes the lateral fusion of thin myofibrils to form mature myofibrils

Fig. 2

Domain organization of a N-RAP and b Krp1. The hatched N-RAP single repeat is encoded by an alternatively spliced exon that is not expressed in cardiac muscle (Mohiddin et al. 2003). The hatched region in the C-terminal super repeat is encoded by an exon reported to be alternatively spliced in skeletal muscle (Gehmlich et al. 2004). Boxes below the diagrams list the known binding partners for each region. See text for details

Fig. 3

Schematic model illustrating N-RAP scaffolding during assembly of the I-Z-I complex (from Manisastry et al. 2009). a Actin filaments form. b Antiparallel N-RAP dimers cross-link actin filaments in an antiparallel orientation. The orientation of each actin filament is indicated by an arrowhead at the pointed end. c α-actinin binds to N-RAP and cross-links the actin filaments. d Krp1 promotes lateral fusion of the Z-bodies to form Z-lines, and N-RAP leaves the complex