Fishing out proteins that bind to titin

Abstract

Another giant protein has been detected in cross-striated muscle cells. Given the name obscurin, it was discovered in a yeast two-hybrid screen in which the bait was a small region of titin that is localized near the Z-band. Obscurin is about 720 kD, similar in molecular weight to nebulin, but present at about one tenth the level (Young et al., 2001). Like titin, obscurin contains multiple immunoglobulin-like domains linked in tandem, but in contrast to titin it contains just two fibronectin-like domains. It also contains sequences that suggest obscurin may have roles in signal transduction. During embryonic development, its localization changes from the Z-band to the M-band. With these intriguing properties, obscurin may not remain obscure for long.

Figure 1.

Diagram of a sarcomere bounded by the Z-bands. The left side of the sarcomere represents a half sarcomere found in vertebrate skeletal myofibrils. Note that the nebulin molecules are part of and extend the entire length of the thin filaments. The right side of the sarcomere reflects a half sarcomere in cardiac muscle cells. The smaller nebulin isoform, nebulette, begins within the Z-band and extends only a short distance along the thin filament. Titin is shown with its NH₂ termini from adjacent sarcomeres overlapping in the Z-band. Groups of three titin filaments are shown aligned together in the half sarcomere and overlapping in the M-band with groups of three from the other half sarcomere. The scale of the drawing does not allow the ratio of six titins per half-thick filament or the two nebulin isoforms per actin thin filament to be illustrated. The double-headed arrows indicate the position of the region of titin used as a bait to pull out obscurin. The M-band is the mid-point of the group of aligned thick myosin filaments (A-band) where obscurin binds in cultured neonatal cardiomyocytes and in adult muscles.