Repetitive amino acid sequences often confer proteins specialized in structure and function, playing a critical role in evolution of proteins. The KSP repeats of neurofilaments medium subunit (NF-M) are heavily phosphorylated, and regulate axonal diameter and neuronal conduction velocity in myelinated axons by extending their molecular backbones. The number of the KSP repeats varies among mammalian species and exhibits a positive correlation with larger axonal diameter in larger animals. However, the role played by differing numbers of KSP repeats in determining cellular structure and function has not been fully examined. Here, we report that a Caenorhabditis-specific protein CNP-2 is a substrate of
tax-6/calcineurin (calcium/calmodulin-dependent serine/threonine phosphatase) and a KSP-like repeat protein rich in Lys, Arg, Ser, Thr, Pro, and Glu, which are frequently found in proteins that display flexible architectural features.
cnp-2 is highly expressed in extensively elastic tissues such as the spermatheca, the spermathecal-uterus valve, and male tail. Also,
cnp-2 is critical for both hermaphrodites and male reproductive processes. Bioinformatics analyses and experiments in transgenic worms revealed that the KSP repeat domain of
unc-89/obscurin (a giant sarcomeric signaling protein) is functionally homologous with CNP-2, although it is distantly related and shows low sequence similarity and identity. In addition, expression of KSP repeats from human NF-M reversed reproduction defects in
cnp-2 worm mutants in a repeat-number-dependent manner; a greater number of KSP repeats showed more efficient effects with respect to morphology and activity. Taken together, the data suggest that
cnp-2 is a KRSTPE-rich repeat protein that shares common features with the KSP repeats of NF-M, including regulatory molecular extension to support inter-molecular interactions, and contractile and elastic cellular structures. This study also provides insight into how alterations in the number of amino acid repeats may play a role in the determination of cellular morphology and function as a molecular mechanism contributing to the evolution of species-specific traits.