Initiation and maturation of I-Z-I bodies in the growth tips of transfected myotubes

While over a dozen I-Z-I proteins are expressed in postmitotic myoblasts an d myotubes it is unclear how, when, or where these first assemble into tran sitory I-Z-I bodies (thin filament/Z-band precursors) and, a short time lat er, into definitive I-Z-I bands. By double-staining the growth tips of tran sfected myotubes expressing (a) MYC-tagged s-alpha-actinins (MYC/s-alpha-ac tinins) or (b) green fluorescent protein-tagged titin cap (GFP/T-cap) with antibodies against MYC and I-Z-I band proteins, we found that the de novo a ssembly of I-Z-I bodies and their maturation into I-Z-I bands involved rela tively concurrent, cooperative binding and reconfiguration of, at a minimum , 5 integral Z-band molecules. These included s-alpha-actinin, nebulin, tit in, T-cap and alpha-actin, Resolution of the similar to 1.0 mu m polarized alpha-actin/nebulin/tropomyosin/troponin thin filament complexes occurred s ubsequent to the maturation of Z-bands into a dense tetragonal configuratio n. Of particular interest is finding that mutant MYC/s-alpha-actinin peptid es (a) lacking spectrin-like repeats 1-4, or consisting of spectrin-like re peats 1-4 only, as well as (b) mutants/fragments lacking titin or alpha-act in binding sites, were promptly and exclusively incorporated into de novo a ssembling I-Z-I bodies and definitive I-Z-I bands as was exogenous full len gth MYC/s-alpha-actinin or GFP/T-cap.