PDZ motifs are modular protein–protein interaction domains, consisting of 80–120 amino acid residues, whose function appears to be the direction of intracellular proteins to multiprotein complexes. In skeletal muscle, there are a few known PDZ-domain proteins, which include neuronal nitric oxide synthase and syntrophin, both of which are components of the dystrophin complex, and actinin-associated LIM protein, which binds to the spectrin-like repeats of α-actinin-2. Here, we report the identification and characterization of a new skeletal muscle protein containing a PDZ domain that binds to the COOH-terminal region of α-actinin-2. This novel 31-kD protein is specifically expressed in heart and skeletal muscle. Using antibodies produced to a fragment of the protein, we can show its location in the sarcomere at the level of the Z-band by immunoelectron microscopy. At least two proteins, 32 kD and 78 kD, can be detected by Western blot analysis of both heart and skeletal muscle, suggesting the existence of alternative forms of the protein. In fact, several forms were found that appear to be the result of alternative splicing. The transcript coding for this Z-band alternatively spliced PDZ motif (ZASP) protein maps on chromosome 10q22.3-10q23.2, near the locus for infantile-onset spinocerebellar ataxia.

We report the identification and characterization of a novel 32-kDa protein expressed in skeletal muscle and located in the Z-disc of the sarcomere. We found that this protein binds to three other Z-disc proteins; therefore, we have named it FATZ, ␥-filamin/ABP-L, ␣-actinin and telethonin binding protein of the Z-disc. From yeast twohybrid experiments we are able to show that the SR3-SR4 domains of ␣-actinin 2 are required to bind the COOH-terminal region of the FATZ as does ␥-filamin/ ABP-L. Furthermore, by using a glutathione S-transferase overlay assay we find that FATZ also binds telethonin. The level of FATZ protein in muscle cells increases during differentiation, being clearly detectable before the onset of myosin. Although FATZ has no known interaction domains, it would appear to be involved in a complex network of interactions with other Z-band components. On the basis of the information known about its binding partners, we could envisage a central role for FATZ in the myofibrillogenesis. After screening our muscle expressed sequence tag data base and the public expressed sequence tag data bases, we were able to assemble two other muscle transcripts that show a high level of identity with FATZ in two different domains. Therefore, FATZ may be the first member of a small family of novel muscle proteins.The Z-disc of vertebrate striated muscle is a region where the antiparallel actin filaments spanning the sarcomere are crosslinked. This supramolecular structure plays an important role in the regulation of contraction both in skeletal and cardiac muscle. Variation of the Z-disc structure is observed during development and differentiation of muscle cells and can be correlated with specific pathological or degenerative conditions associated with muscle injuries or atrophies.The number of different protein components of the Z-disc is far from being complete, and many of the newly discovered muscle proteins appear to be localized in this region, and very generally they can be divided into two groups based on their location. In the first group are proteins that are only partially in the Z-disc while extending into other portions of the sarcomere or the sarcolemma. An example of the first group is titin that acts as a ruler for the ordered distribution of sarcomeric proteins and is particularly important in Z-disc assembly. The NH 2 -terminal portion of titin extends into the Z-disc, where two different sub-domains have been shown to interact specifically with ␣-actinin (1-3) and also with the muscle-specific protein telethonin (4). Although telethonin binds to the NH 2 -terminal domain of titin, it also acts as one of the substrates of the titin serine kinase domain that is located outside of the Z-disc (5). The second group is composed of proteins, many of which have been recently discovered and characterized, that appear to be entirely located in the Z-disc (e.g. ␣-actinin, Nspl1, telethonin, ZASP1, and CapZ). Among these proteins, ␣-actinin plays a central role by directly cross-linking the actin molecules. ...