Rng3, a member of the UCS family of myosin co-chaperones, associates with myosin heavy chains cotranslationally

The production of functional myosin heavy chains in many eukaryotic organisms requires the function of proteins containing UCS domains (UNC-45/CRO1/She4), which bind to the myosin head domain and stimulate its folding. UCS proteins are essential for myosin-related functions such as muscle formation, RNA localization and cytokinesis. Here, we show that the Schizosaccharomyces pombe UCS protein Rng3 associates with polysomes, suggesting that UCS proteins might assist myosin folding cotranslationally. To identify Rng3 cotranslational targets systematically, we purified Rng3-associated RNAs and used DNA microarrays to identify the transcripts. Rng3 copurified with only seven transcripts (around 0.1% of S. pombe genes), including all five messenger RNAs encoding myosin heavy chains. These results suggest that every myosin heavy chain in S. pombe is a cotranslational target of Rng3. Furthermore, our data suggest that microarray-based approaches allow the genome-wide identification of cotranslational chaperone targets, and thus pave the way for the dissection of translation-linked chaperone networks.