Replicating myoblasts and fused myotubes express the calcium-regulated proteins S100A1 and S100B

We investigated the expression and the subcellular localization of S100A1 a nd S100B, two Ca2+-binding proteins of the EF-hand type, in replicating myo blasts and fused myotubes. Northern blot and reverse transcriptase-polymera se chain reaction analyses revealed the presence of S100A1 mRNA and S100B m RNA respectively, in myoblasts. Immunofluorescence and immunogold electron microscopy were used to localize individual proteins in myoblasts and myotu bes. In the present report we document that: (1) in replicating myoblasts S 100B is localized to intracellular membranes, including Golgi membranes, vi mentin intermediate filaments (IFs) and microtubule (MT) structures; (2) in the same cells S100A1 is found associated with intracellular membranes; (3 ) following treatment of replicating myoblasts with colchicine, a fraction of S100B remains colocalized with bundled and collapsed vimentin Ifs, where as another fraction follows the destiny of endoplasmic membranes; (4) under the same conditions S100A1, like a fraction of S100B, follows the collapse of the endoplasmic reticulum around the nucleus; and (5) in fused myotubes S100A1 is found diffusely in the cytoplasm, whereas S100B is mostly found associated with vimentin Ifs. These data suggest that in the skeletal myoge nic cell line used in the present study S100A1 and S100B might share bindin g sites on or close to intracellular membranes, but display a significant d egree of target specificity with respect to Ifs and MTs. The results of the se analyses suggest that expression of S100B in skeletal muscle cells may b e developmentally regulated and lend support to the possibility that S100B might regulate the MT and IF dynamics.