Expression and localisation of annexin VII (synexin) isoforms in differentiating myoblasts

Annexin VII exists in a 47 kDa and a 51 kDa isoform with the 51 kDa protein being the only isoform present in skeletal muscle. Expression of the 51 kD a isoform during myogenesis and localization was studied in cells after con version into myogenic cells by transduction with MyoD and in mouse and huma n myogenic cell lines. MyoD expression in NIH3T3 and C3H10T1/2 fibroblasts led to disappearance of the mRNA specific for the 47kDa isoform and appeara nce of the 51 kDa isoform-specific mRNA. The overall amount of annexin VII protein was reduced in myogenic converted cells. Both in undifferentiated a nd differentiated cells annexin VII was localized by immunofluorescence mic roscopy to punctate structures which were distributed all over the cell. A GFP annexin VII fusion protein showed a similar distribution. Cell fraction ation studies indicated that annexin VII is equally distributed between cyt osol and membrane fractions in undifferentiated cells, while in differentia ted cells it is exclusively present in the membrane fraction. By sucrose gr adient centrifugation of postnuclear supernatants we identified two distinc t annexin VII-containing membrane populations that cofractionated with cave olin 3- and sorcin-containing membranes.