The small GTPase protein Rac1 is involved in a wide range of biological processes, yet its role in cell differentiation is mostly unknown. Here we show that Rac1 activity is high in proliferating myoblasts and decreases during the differentiation process. To analyze the involvement of Rac1 in muscle differentiation, different forms of the protein were expressed in muscle cells. A constitutively activated form of Rac1 (Rac1Q61L) inhibited the activity of MyoD in promoting muscle differentiation, whereas a dominant negative form of Rac1 (Rac1T17N) induced the activity of MyoD in promoting muscle differentiation. Expression of Rac1T17N imposed myogenic differentiation on myoblasts growing under mitogenic conditions. In inquiring whether Rac1 affected the withdrawal of myoblasts from the cell cycle, we analyzed the expression of cyclin D1 and p21 The Rho family of small GTP-binding proteins, which includes Rho, Rac, and CDC42, is involved in a wide range of biological processes, including cell motility, cell adhesion, cell morphology, cytokinesis, and cell proliferation (for reviews, see Refs. 1 and 2).In mammalian cells, Rac functions by generating the actinrich lamellipodial protrusions and membrane ruffling that are thought to be a major part of the driving force for cell movement (3). Independent of this effect of cytoskeletal rearrangements, Rac1 and CDC42 induce the activation of mitogenactivated protein kinase cascades. Some reports suggest that Rac1 and CDC42 activate the c-Jun N-terminal kinase (JNK) 1 and the p38 MAPK (4 -7), whereas other studies indicate that Rac1 cross-talks with the ERK MAPK pathway. Rac and CDC42 can synergize with Raf1 or MEK1 to promote ERK activation (8). It turns out that p21-activated kinase (PAK), which is a CDC42 and Rac target, positively regulates Raf1 and MEK1 activities through phosphorylation of specific serine residues (9, 10). These functions of Rac and CDC42 in cytoskeletal reorganization and the induction of MAPK signaling pathways may explain their role in the regulation of a variety of cellular processes in development and morphogenesis (11).The differentiation of skeletal muscle cells involves two major stages: (a) withdrawal of myoblasts from the cell cycle, and (b) subsequent expression of myotube-specific genes. Proliferating myoblasts express two myogenic transcription factors from the basic helix-loop-helix family, MyoD and Myf5, before the onset of muscle differentiation (12)(13)(14). Once activated, MyoD and Myf5 induce the withdrawal of myoblasts from the cell cycle and the expression of another myogenic basic helixloop-helix factor, myogenin, as well as transcription factors from the MEF2 family. Together, myogenin and MEF2 family members cooperate in the activation of many muscle structural genes (15,16).Although the molecular mechanisms controlling myogenesis at the transcriptional level are well characterized, the signaling molecules that mediate the transduction of extracellular cues and affect the muscle regulatory factors are only partially uncovered....
