Interestingly, during the period between 10 hpf and 24 hpf, zebrafish somites and heart are forming, such that by 24 hpf a beating heart can be observed. These data provide evidence for the expression of both alternative splice forms of Ube4b during this period of zebrafish development in a sequential pattern like that seen in mammalian myogenesis. Only yeast coexpressing UFD2a and VCP/p97 fusion proteins grew on medium lacking histidine in the presence of 3- aminotriazole, a histidine biosynthesis inhibitor, indicating an interaction between UFD2a and VCP/p97 fusion proteins. In contrast, neither yeast coexpressing UFD2a-7/7a and VCP/p97 fusion proteins nor yeast expressing bait or prey proteins alone grew in this medium. The lack of interaction between UFD2a-7/7a and VCP/p97 was confirmed by the absence of b – galactosidase activity in a liquid assay. We describe here 2 muscle-specific alternative splice forms of the ubiquitin ligase, UFD2a. The smaller, UFD2a-7 contains the previously identified exon 7, while UFD2a-7/7a incorporates a novel exon 7a in addition to exon 7. Both of these alternatively spliced exons are located just downstream of the previously described MPAC regulatory domain and within the N-terminal extension of UFD2a, which is unique to vertebrate orthologs. The GenBank database includes expressed sequence tag entries containing exon 7 spliced to exon 8 in human primary fibroblasts and thalamus tissue, and a full-length cDNA sequence of UFD2a-7 cloned from a mouse fetal brain library. However, we did not detect UFD2a-7 protein in human or mouse fibroblast or brain tissues. Two recent genome-wide exon-junction and whole transcript microarrays, which examined the expression of alternative pre-mRNA splice forms across a total of 92 tissues and cell lines, both found that exon 7 was not present in UFD2a cDNA from human fetal brain tissue. In fact, consistent with the data presented here, exon 7 was exclusively expressed in adult human skeletal BAY 43-9006 muscle, heart, tongue and to a lower extent peripheral blood leukocytes. Of the nine fetal tissues tested, only fetal heart expressed exon 7. The tissue specificity of exon 7a was not examined since its presence in UFD2a had not yet been reported. Our RT-PCR and Western blot data suggest that UFD2a-7 is WY 14643 uniquely expressed in a transient manner during myogenic development and myoblast differentiation. The timing of UFD2a-7 expression during differentiation and after injury appeared to correlate with that of the myogenic regulatory factors Myf5 and MyoD. Interestingly, ubiquitin-dependent degradation regulates most of these myogenic regulatory factors during myogenesis. The degradation of Myf5, in particular, is required for myoblast fusion which occurs at the time of UFD2a-7 expression. In developing skeletal muscle, it coincided with the greatest expression of embryonic MHC. In cell culture models of differentiation and in vivo models of regeneration,