Muscle Z-band ultrastructure: titin Z-repeats and Z-band periodicities do not match

Vertebrate muscle Z-bands show zig-zag densities due to different sets of alpha-actinin cross-links between anti-parallel actin molecules. Their axial extent varies with muscle and fibre type: approximately 50 nm in fast and approximately 100 nm in cardiac and slow muscles, corresponding to the number of alpha-actinin cross-links present. Fish white (fast) muscle Z-bands have two sets of alpha-actinin links, mammalian slow muscle Z-bands have six. The modular structure of the approximately 3 MDa protein titin that spans from M-band to Z-band correlates with the axial structure of the sarcomere; it may form the template for myofibril assembly. The Z-band-located amino-terminal 80 kDa of titin includes 45 residue repeating modules (Z-repeats) that are expressed differentially; heart, slow and fast muscles have seven, four to six and two to four Z-repeats, respectively. Gautel et al. proposed a Z-band model in which each Z-repeat links to one level of alpha-actinin cross-links, requiring that the axial extent of a Z-repeat is the same as the axial separation of alpha-actinin layers, of which there are two in every actin crossover repeat. The span of a Z-repeat in vitro is estimated by Atkinson et al. to be 12 nm or less; much less than half the normal vertebrate muscle actin crossover length of 36 nm. Different actin-binding proteins can change this length; it is reduced markedly by cofilin binding, or can increase to 38.5 nm in the abnormally large nemaline myopathy Z-band. Here, we tested whether in normal vertebrate Z-bands there is a marked reduction in crossover repeat so that it matches twice the apparent Z-repeat length of 12 nm. We found that the measured periodicities in wide Z-bands in slow and cardiac muscles are all very similar, about 39 nm, just like the nemaline myopathy Z-bands. Hence, the 39 nm periodicity is an important conserved feature of Z-bands and either cannot be explained by titin Z-repeats as previously suggested or may correlate with two Z-repeats.