Tandemly Arranged Repeats of a Novel Highly Charged 16-Amino-Acid Motif Representing the Major Component of the Sperm-Tail-Specific Axoneme-Associated Protein Family Dhmst101 form Extended alpha-Helical Rods within the Extremely Elongated Spermatozoa of Drosophila hydei

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Abstract
We have previously reported that the sperm-tail-specific proteins in Drosophila hydei, encoded by the small Dhmst101 gene family, contained several tandem repeats of a novel highly charged, well-conserved cysteine-containing motif of 16 amino acids, KKKCAEAAKKEKEAAE [Neesen, J., Bünemann, H. & Heinlein, U. A. O. (1994) Dev. Biol. 162, 414-425] and suggested that this motif might be important in the structural and functional integrity of the sperm tail. We tested this suggestion by examining structure formation by model synthetic peptides containing the 16-residue sequence and corresponding peptides with one and two repeats of the sequence with Cys being replaced by Ala. We find that all these peptides form monomeric alpha-helices and that the helix content is considerably enhanced as the number of tandem repeats increases. These results are consistent with tandemly arranged 16-amino-acid repeats in Dhmst101 proteins forming extended alpha-helical rods, with the highly conserved Cys present in each 16-amino-acid motif being involved in regular interhelical cross-linking, thus providing a rigid, stable framework within the extremely elongated spermatozoa of Drosophila hydei.