Zasp/Cypher internal ZM-motif containing fragments are sufficient to co-localize with alpha-actinin--analysis of patient mutations.

Z-band alternatively spliced PDZ-containing protein (ZASP/Cypher) has an important role in maintaining Z-disc stability in striated and cardiac muscle. ZASP/Cypher interacts through its PDZ domain with the major Z-disc actin cross-linker, alpha-actinin. ZASP/Cypher also has a conserved sequence called the ZM-motif, and it is found in two alternatively spliced exons 4 and 6. We have shown earlier that the ZM-motif containing internal regions of two related proteins ALP and CLP36 interact with alpha-actinin rod region, and that the ZM-motif is important in targeting ALP to the alpha-actinin containing structures in cell. Here, we show that the ZASP/Cypher internal fragments containing either ZM exon 4 or 6 co-localized with alpha-actinin in cultured myoblasts and nonmuscle cells. Fragments of 130 residues around the ZM-consensus were sufficient for localization, which is similar to our previous results of ALP. Moreover, ZASP/Cypher protein interacted directly with the alpha-actinin rod and competed with ALP in binding to the rod. During the inhibition of stress fiber assembly ZASP/Cypher and alpha-actinin co-localization could be partially disturbed, suggesting that ZASP/Cypher is bound to alpha-actinin mainly when alpha-actinin is localizing in stress fibers. Many point mutations found in cardiomyopathy patients are located in the internal region of ZASP/Cypher. However, we found no evidence that human patient mutations in the internal domain would affect the ZASP/Cypher co-localization with alpha-actinin, or that the mutations would destabilize the ZASP/Cypher protein.