The UNC-45 or small muscle protein 1 of C.elegans is expressed in two forms from different genomic positions in mammals, as a general tissue protein UNC-45a and a specific form Unc-45b expressed only in striated and skeletal muscle. All members carry up to three amino-terminal tetratricopeptide repeat (TPR) domains towards their N-terminal, a UCS domain at the C-terminal that contains a number of Arm repeats Pfam:PF00514 and this central region of approximately 400 residues. Both the general form and the muscle form of UNC-45 function in myotube formation through cell fusion. Myofibril formation requires both GC and SM UNC-45, consistent with the fact that the cytoskeleton is necessary for the development and maintenance of organised myofibrils [1]. The S. pombe Rng3p, is crucial for cell shape, normal actin cytoskeleton, and contractile ring assembly, and is essential for assembly of the myosin II-containing progenitors of the contractile ring. Widespread defects in the cytoskeleton are found in null mutants of all three fungal proteins [2]. Mammalian Unc45 is found to act as a specific chaperone during the folding of myosin and the assembly of striated muscle by forming a stable complex with the general chaperone Hsp90. The exact function of this central region is not known [3].

1: Two mammalian UNC-45 isoforms are related to distinct cytoskeletal and muscle-specific functions. Price MG, Landsverk ML, Barral JM, Epstein HF; J Cell Sci. 2002;115:4013-4023. PMID:12356907

2: Yeast UCS proteins promote actomyosin interactions and limit myosin turnover in cells. Lord M, Sladewski TE, Pollard TD; Proc Natl Acad Sci U S A. 2008;105:8014-8019. PMID:18523008

3: Unc45b forms a cytosolic complex with Hsp90 and targets the unfolded myosin motor domain. Srikakulam R, Liu L, Winkelmann DA; PLoS ONE. 2008;3:e2137. PMID:18478096