EXPRESSION AND LOCALIZATION OF RAP1 PROTEINS DURING MYOGENIC DIFFERENTIATION

The RAP1 subfamily of small GTPases has been involved in various diffe rentiation programs. In skeletal muscle, several lines of evidence sug gest that various small GTPases could be implicated in muscle developm ent. This raised the question of whether the RAP1 proteins (RAP1A and/ or RAP1B) could be involved in myogenesis. In the present study, we re port on the regulation of RAP1 transcripts and proteins during myogeni c differentiation. Northern blot analysis performed with differentiate d and undifferentiated C2 myogenic cells pointed out that both genes u ndergo specific regulation during myogenesis in vitro since differenti ation of C2 cells was accompanied by a down-regulation of RAP1B gene t ranscription and continuous expression of the RAP1A mRNA. In addition, immunofluorescence experiments revealed the accumulation of the RAP1 proteins in differentiated C2 cells and in primary culture of mouse my otubes. Investigation of the intracellular location of RAP1 proteins i n undifferentiated and differentiated C2 cells showed that the protein s were associated with the late endocytic compartments. To verify that the build-up of RAP1 proteins had a relevance for developmental mecha nisms in vivo, we studied their expression and localization at differe nt stages of skeletal muscle development. We found that RAP1 proteins accumulated in specialized muscle cell domains undergoing important mo difications during early and late myogenesis: these were the neuromusc ular and myotendinous junctions, respectively. Altogether, our data in dicate that RAP1 proteins are regulated during myogenic differentiatio n.