MuRF1 is a member of the RBCC (RING, B-box, coiled-coil) superfamily that has been proposed to act as an atrogin during muscle wasting. Here, we show that MuRF1 is preferentially induced in type-II muscle fibers after denervation. Fourteen days after denervation, MuRF1 protein was further elevated but remained preferentially expressed in type-II muscle fibers. Consistent with a fiber-type dependent function of MuRF1, the tibialis anterior muscle (rich in type-II muscle fibers) was considerably more protected in MuRF1-KO mice from muscle wasting when compared to soleus muscle with mixed fiber-types. We also determined fiber-type distributions in MuRF1/MuRF2 double-deficient KO (dKO) mice, because MuRF2 is a close homolog of MuRF1. MuRF1/MuRF2 dKO mice showed a profound loss of type-II fibers in soleus muscle. As a potential mechanism we identified the interaction of MuRF1/MuRF2 with myozenin-1, a calcineurin/NFAT regulator and a factor required for maintenance of type-II muscle fibers. MuRF1/MuRF2 dKO mice had lost myozenin-1 expression in tibialis anterior muscle, implicating MuRF1/MuRF2 as regulators of the calcineurin/NFAT pathway. In summary, our data suggest that expression of MuRF1 is required for remodeling of type-II fibers under pathophysiological stress states, whereas MuRF1 and MuRF2 together are required for maintenance of type-II fibers, possibly via the regulation of myozenin-1. (C) 2010 Elsevier Inc. All rights reserved.