Proteins with tandemly arranged repeats of a highly charged 16-amino-acid motif encoded by the Dhmst101 gene family are structural components of the outer sheath of the extremely elongated sperm tails of Drosophila hydei

Fruit fly species of the genus Drosophila show a remarkable variation in sp erm length. Some of them produce gigantic sperm several times the total mal e body length. Sperm of Drosophila hydei, for example, are more than 20 mm long. Little is known about the advantage of such elongated sperm or about the proteins that stabilize their thin flagellar tails. Recently, two membe rs of a novel gene family Dhmst101(1) and Dhmst101(2), whose gene products are associated with the sperm tail, were isolated and characterized. Here a third member of this gene family, Dhmst101(3), is described. It was previo usly demonstrated that ail three genes are located in a single small cluste r on chromosome 5 of D. hydei They are located within 15 kb of genomic DNA, oriented in the same direction and transcribed testis-specifically. The en coded sperm tail-specific proteins are mainly composed of tandemly arranged repeats of a highly charged, cysteine-containing motif of 16 amino acids w ith the consensus sequence KKKCA/EEAAKKEKEAAE. Experiments with synthetic r epeat monomers and dimers have demonstrated a tendency for alpha-helical ro d formation, which increased strongly with an increase in repeat number. Th erefore, Dhmst101 proteins with 7-60 repeats with regularly spaced cystein- residues are thus expected to form long alpha-helical rods cross-linked by numerous Cys-Cys bridges. Here we apply immunoelectron microscopy and monos pecific antibodies, alpha-mst101, raised against the KKKCAEAAKKEKEAAE-motif to investigate the distribution of Dhmst101 proteins within the sperm tail of D, hydei. We show that Dhmst101 proteins are part of the outer sheath o f the sperm tail where they presumably help to provide a tight but elastic envelope for the extremely extended spermatozoa of D. hydei.