Tissue expression and actin binding of a novel N-terminal utrophin isoform.

Utrophin and dystrophin present two large proteins that link the intracellular actin cytoskeleton to the extracellular matrix via the C-terminal-associated protein complex. Here we describe a novel short N-terminal isoform of utrophin and its protein product in various rat tissues (N-utro, 62  kDa, amino acids 1-539, comprising the actin-binding domain plus the first two spectrin repeats). Using different N-terminal recombinant utrophin fragments, we show that actin binding exhibits pronounced negative cooperativity (affinity constants K(1) = -5 × 10(6) and K(2) =-1 × 10(5 )M(-1)) and is Ca(2+)-insensitive. Expression of the different fragments in COS7 cells and in myotubes indicates that the actin-binding domain alone binds exclusively to actin filaments. The recombinant N-utro analogue binds in vitro to actin and in the cells associates to the membranes. The results indicate that N-utro may be responsible for the anchoring of the cortical actin cytoskeleton to the membranes in muscle and other tissues.