Analysis of the flagellar apparatus in the motile cells of Allomyces macrogynus

Abstract

Allomvces macroavnus is characterized by posteriorly uniflagellated reproductive cells. The flagellar apparatus consists of a functional kinetosome, nonfunctional centriole, anchoring struts, basal mitochondrion, and a root system consisting of a rhizoplast and a microtubular root. This study was undertaken to more clearly understand the relationship between flagellar apparatus structure and function, with emphasis placed on the flagellar root system. The specific aims of this study were: 1) Reconstruct from transmission electron micrographs the structure of the flagellar apparatus. 2) Determine the possible function(s) of the rhizoplast 3) Determine the developmental sequence of the rhizoplast during sporogenesis. 4) Determine whether α-tubulin of the microtubular root is acetylated. The structural observations make it likely that the rhizoplast may allow the flagellum to determine the direction of motility since the rhizoplast is attached directly to the a- and c- tubules of the kinetosome triplets 1, 2, and 9. The rhizoplast also functions to link the single basal mitochondrion to the kinetosome. This establishes an inherent polarity in the cell so that the plane of flagellar beat is always oriented in the same direction with respect to the internal organization of the cell. The rhizoplast may also function to align the flagellar apparatus of the male and female gametes following fusion. The microtubular root is resistant to microtubule depolymerizing drugs. Two-dimensional gel electrophoresis demonstrates the presence of α-1 and α-3 tubulin isoforms in axonemal and flagellar root microtubules, suggesting that both sets of microtubules may exist primarily in an acetylated (α-3) state. This was confirmed by cross reaction of tubulin from axonemes and the microtubular root with antibodies (gift from G. Piperno) directed against acetylated a-tubulin. This acetylation may contribute to the stability of the microtubular root. Development of the rhizoplast occurs after the onset of sporogenesis. The rhizoplast is fully formed immediately prior to cleavage of the sporangium. Evidence is presented that a specific region of the nuclear membrane (nucleus-associated organelle) is instrumental in aligning the kinetosome to the nucleus, associated with the forming rhizoplast, and possibly functions to help orient the basal mitochondrion to the kinetosome.