Heterogeneity of Z-band structure within a single muscle sarcomere: implications for sarcomere assembly.
The vertebrate striated muscle Z-band connects actin filaments of opposite polarity from adjacent sarcomeres and allows tension to be transmitted along a myofibril during contraction. Z-bands in different muscles have a modular structure formed by layers of alpha-actinin molecules cross-linking actin filaments. Successive layers occur at 19 nm intervals and have 90 degrees rotations between them. 3D reconstruction from electron micrographs show a two-layer "simple" Z-band in fish body fast muscle, a three-layer Z-band in fish fin fast muscle, and a six-layer Z-band in mammalian slow muscle. Related to the number of these layers, longitudinal sections of the Z-band show a number of zigzag connections between the oppositely oriented actin filaments. The number of layers also determines the axial width of the Z-band, which is a useful indicator of fibre type; fast fibres have narrow (approximately 30-50 nm) Z-bands; slow and cardiac fibres have wide (approximately 100-140 nm) Z-bands. Here, we report the first observation of two different Z-band widths within a single sarcomere. By comparison with previous studies, the narrower Z-band comprises three layers. Since the increase in width of the wider Z-band is about 19 nm, we conclude that it comprises four layers. This finding is consistent with a Z-band assembly model involving molecular control mechanisms that can add additional layers of 19 nm periodicity. These multiple Z-band structures suggest that different isoforms of nebulin and titin with a variable number of Z-repeats could be present within a single sarcomere.
Luther
Pradeep K
PK
Biological Structure and Function Section, Division of Biomedical Sciences, Faculty of Medicine, Imperial College London, Exhibition Road, London SW7 2AZ, UK. p.luther@imperial.ac.uk
Padrón
Raúl
R
Ritter
Stephan
S
Craig
Roger
R
Squire
John M
JM
D000185:Actinin / Q000378:metabolism; D000199:Actins / Q000737:chemistry; D000818:Animals; D064211:Connectin; D009124:Muscle Proteins / Q000737:chemistry; D011494:Protein Kinases / Q000737:chemistry; D011894:Rana pipiens; D012518:Sarcomeres / Q000737:chemistry / Q000378:metabolism / Q000648:ultrastructure*
D000199:Actins; D064211:Connectin; D009124:Muscle Proteins; D000185:Actinin; D011494:Protein Kinases