UCS proteins – versatile chaperones bringing and keeping myosin in shape
In contrast to Hsp70 and Hsp90, UCS proteins are specialized chaperones that act exclusively on myosin substrates. UCS proteins are expressed in all eukaryotes and exhibit a remarkably large spectrum of chaperone activities. Fungal UCS proteins (e.g. P. anserina Cro1, S. cerevisiae She4 and S. pombe Rng3) are critical for a variety of acto-myosin dependent processes such as cytokinesis, mRNA transport and endocytosis. As indicated by their functional versatility, the fungal UCS proteins can interact with different myosin classes and contribute to their stability. UCS proteins of metazoans, so-called UNC-45 proteins, are essential for cell proliferation, cytokinesis and for organizing thick filaments in muscle sarcomeres. In contrast to their fungal counterparts, UNC-45 has an additional N-terminal TPR (tetratricopeptide repeat) domain allowing for collaboration with Hsp70 and Hsp90. Moreover, the substrate specificity of UNC-45 proteins appears to be more pronounced, as an interaction has been shown only for myosin type II forms.
Our findings: 2013 (UNC-45 composes a myosin assembly line)
2013 An assembly line for myosin filaments, set up by UNC-45
The crystal structure of C. elegans UNC-45 did not only reveal the overall organization of TPR, central, neck and UCS domains in forming a mouth-like chaperone structure, but, most importantly, uncovered the ability of UNC-45 proteins to self-assemble linear, polar protein filaments. Accordingly, UNC-45 is able to adopt a similar filamentous structure as its native myosin substrate. The backbone of the observed UNC-45 filament is formed by the central and neck domains leaving the functional TPR and UCS domains free to interact with partner chaperones and myosin substrates, respectively. The crystallized UNC-45 filament could be also observed in solution by employing a directed photo-crosslinking methodology. This approach demonstrated the transient and concentration dependent formation of short UNC-45 chains, having 2-5 subunits. Strikingly, the patterning function of UNC-45 during muscle development could be confirmed in vivo. Upon expressing UNC-45 mutants that specifically impede chain formation, a dominant-negative effect was observed in wild-type worms. The expressed mutants disturbed thick filament formation and compromised sarcomere integrity.
In conclusion, UNC-45 assembles a multimeric scaffolding complex that offers binding sites for Hsp70 and Hsp90 chaperones in a precisely defined pattern to work on the array of myosin head domains protruding from pre-arranged myofilaments. In fact, the periodicity of UNC-45 molecules within the chaperone chain is similar to the spacing of adjacent myosin heads along the thick filaments.