Structure-function relationships in dystrophin and utrophin.

Introduction The integrity of the animal cell membrane is believed to be maintained in part by the large (approx. 400 kDa) cytoskeletal proteins dystrophin and utrophin. Both proteins are capable of forming a link, probably flexible and potentially extensible/compressible, between the actin cytoskeleton and the cell membrane itself. Utrophin is expressed in all cell types, whereas dystrophin expression is restricted to muscle and tissues of neuronal origin. Mutations in these genes, utrophin on human chromosome 6 and dystrophin on the X chromosome, would be expected to have severe consequences for the cell. Mutations in the dystrophin gene are known, giving rise to the Duchenne and Becker muscular dystrophy phenotypes (DMD and BMD respectively). Mutations that lead to the functional absence of dystrophin protein promoter defects, premature stop codons and other missense mutations generally present as the more common and severe DMD phenotype. Deletions, truncations and substitutions that maintain the reading frame of the DNA and allow the expression of altered dystrophin protein generally lead to the milder but less common BMD phenotype. These rarer mutations are therefore much more informative in ascribing functional importance to the dystrophin protein, as disease severity can be correlated with the position of any mutations in the protein. Despite the correspondingly large size of the utrophin gene, no mutations in utrophin have yet been found, leading to the suggestion that the protein is essential for life. An understanding of the functions of the various regions of the dystrophin protein and its close relative, utrophin, will provide useful information regarding the potential use of utrophin in gene therapy approaches in DMD and BMD. Dystrophin and utrophin share considerable sequence and presumed functional similarity; Structure-function relationships in dystrophin and utrophin S. J. Winder MRC Centre, Hills Road, Cambridge CB2 2QH, U.K. 497 each possesses an N-terminal F-actin-binding domain, a series of triple-helical coiled-coil repeats making up approx. 75% of each protein and a C-terminal region comprising several domains involved in protein-protein interactions and possibly signalling functions. In-frame deletions in the central triple-helical coiled-coil regions, by far the most common, present with the mildest BMD phenotypes, whereas mutations in the N-terminal region present with severe BMD and mutations in the C-terminal region lead to DMD. On this basis it can be seen that the regions in dystrophin that form the connections to the actin cytoskeleton and to the membrane are the most important, with the central (spacer/flexible) region to some extent being redundant. The present paper will therefore concentrate on recent structural and functional analysis of the Nand C-terminal domains, with details of the central coiled-coil region presented elsewhere [ 1,2].