[COMPARATIVE STUDY OF Hrs AND OTHER ENDOSOMAL MARKERS CELLULAR LOCALIZATION IN DROSOPHILA MELANOGASTER...

Acrosome is a special organelle in spermatozoids necessary for fertilizing oocyte and originates, according to various theories, either from Golgi apparatus, or from endosomes and lysosomes. One of the proteins, found at mammalian acrosome, is Hgs, a homologue of Drosophila melanogaster Hrs (Hepatocyte growth factor regulated tyrosine kinase substrate), a known marker of multivesicular bodies (MVBs). However, although Drosophila acrosome was extensively studied, it is yet unknown whether Hrs localizes at acrosome similar to Hgs and, more generally, whether the spectrum of acrosomal proteins in Drosophila is the same as in mammals. Hrs (hepatocyte growth factor regulated tyrosine kinase substrate) is the multidomain vesicular protein participating in the endosome-lysosome protein sorting. We demonstrated that two protein variants of the Drosophila Hrs are expressed in testes: a longer isoform B, and a shorter isoform A, which lacks VHS and FYVE domains that are necessary for anchoring Hrs in endosomes. We found that Hrs isoform B is concentrated at fusoma of spermatocytes in contrast to mammalian Hrs. This localization requires the C-terminus of the protein, starting from the aminoacid residue 383. In situ hybridization of hrs RNA probe showed that the gene is expressed early in spermatogenesis consistently with Hrs localization in early fusoma. Additionally, we demonstrated that Hrs is dispensable for cytokinesis. Finally, it was found that although Drosophila Hrs does not localize at acrosome, the other endosomal markers--Rab4, Rab7, and Rab11--are detected at the organelle.