Identification of Caenorhabditis elegans K02H8.1 (CeMBL), a functional ortholog of mammalian MBNL proteins.

The genome of the nematode Caenorhabditis elegans possesses an orthologous sequence to the Drosophila muscleblind (mbl) and mammalian muscleblind-like genes (MBNLs). This ortholog, K02H8.1, which has a high degree of homology (about 50%) to human MBNLs, encodes two zinc finger domains, as does the sequence of the Drosophila mbl gene. This distinguishes it from human MBNLs, which encode four zinc finger domains. In this study, we cloned six major isoforms of K02H8.1 using cDNA generated from C. elegans total RNA. All six of the cloned isoforms had an SL1 leader sequence at the 5'-position. Interestingly, one of the isoforms lacked a zinc finger domain-encoding sequence. To understand better the function of K02H8.1, we performed yeast three-hybrid experiments to characterize the binding of K02H8.1 to bait RNAs. K02H8.1 exhibited strong binding affinity for CUG and CCUG repeats, and the binding affinity was very similar to that of MBNLs. In addition, promoter analysis was performed using promoter-green fluorescent protein (GFP) fusion constructs. The expression of GFP driven by the K02H8.1 promoter was absent in muscle; however, significant GFP expression was detected in the neurons around the pharynx.