We previously showed that alternatively spliced ankyrins-G, the Ank3 gene products, are expressed in skeletal muscle and localize to the postsynaptic folds and to the sarcoplasmic reticulum. Here we report the molecular cloning, tissue expression, and subcellular targeting of Ank G107 , a novel ankyrin-G from rat skeletal muscle. Ank G107 lacks the entire ANK repeat domain and contains a 76-residue sequence near the COOH terminus. This sequence shares homology with COOH-terminal sequences of ankyrins-R and ankyrins-B, including the muscle-specific skAnk1. Despite widespread tissue expression of Ank3, the 76-residue sequence is predominantly detected in transcripts of skeletal muscle and heart, including both major 8-and 5.6-kb mRNAs of skeletal muscle. In 15-day-old rat skeletal muscle, antibodies against the 76-residue sequence localized to the sarcolemma and to the postsynaptic membrane and crossreacted with three endogenous ankyrins-G, including one 130-kDa polypeptide that comigrated with in vitro translated Ank G107 . In adult muscle, these polypeptides appeared significantly decreased, and immunofluorescence labeling was no more detectable. Green fluorescent protein-tagged Ank G107 transfected in primary cultures of rat myotubes was targeted to the plasma membrane. Deletion of the 76-residue insert resulted in additional cytoplasmic labeling suggestive of a reduced stability of Ank G107 at the membrane. Recruitment of the COOH-terminal domain to the membrane was much less efficient but still possible only in the presence of the 76-residue insert. We conclude that the 76-residue sequence contributes to the localization and is essential to the stabilization of Ank G107 at the membrane. These results suggest that tissue-dependent and developmentally regulated alternative processing of ankyrins generates isoforms with distinct sequences, potentially involved in specific protein-protein interactions during differentiation of the sarcolemma and, in particular, of the postsynaptic membrane.

We have previously shown that long-term treatment of primary cultured astrocytes with TGF beta 1 induces morphological changes accompanied by increases in actin and GFAP synthesis, and a profound rearrangement of the cytoskeleton. The present report describes the short-term reorganization of actin filaments induced by TGF beta 1 in rat cerebellum cultured astrocytes and in an astrocytic cell line. TGF beta 1 caused the appearance of new actin and vinculin organizations, without protein synthesis. This cytoskeletal rearrangement was followed by altered cell-cell interactions. All these changes induced by TGF beta 1 were different and slower than those induced by serum, PDGF, and endothelin. TGF beta 1 induced the appearance of lamellipodia, organelles found at the cell front of motile cells in low-density cultures of immortalized astrocytes. These results indicate that the changes in the astrocyte cytoskeleton induced by TGF beta 1 are probably associated with cell movement. The events promoted by TGF beta 1 might help to clarify its action in the brain during embryogenesis and in tissue repair.