Towards a Molecular Map of the Algal Basal Apparatus

Abstract

The flagellar apparatus of flagellate algal cells is a complex cytoskeletal system consisting of several hundred polypeptides which together form the flagella (one to sixteen depending on the cell type) and the structures anchoring the flagella inside the cell, termed the basal apparatus. The basal apparatus comprises the basal bodies and fibrous material associated with the basal bodies. It is thought to be the functional equivalent of the centrosome of other eukaryotic cells since it is the main microtubule organizing center (MTOC; Pickett-Heaps, 1969) of the flagellate cell: the basal bodies function as a template for the assembly of the axoneme (the principal cytoskeletal component of a flagellum); additionally from the basal bodies originate the flagellar root microtubules which extend throughout the cell and mediate interactions with different cell organelles (Melkonian, 1984). Often these root microtubules also organize additional cytoskeletal microtubules which are collectively termed ‘secondary cytoskeletal microtubules’ (Pickett-Heaps, 1975). The function of the basal apparatus of algal cells as an organizer of the mitotic spindle (the genuine function of centrosomes; Kalnins, 1992) is less certain, during mitosis the basal apparatuses are often near or at the spindle poles but in some algal taxa they are located far from the spindle poles (Mattox and Stewart, 1984). Despite their common function, the structure of centrosomes varies considerably within eukaryotes, and as examples we refer to the trilamellar spindle pole bodies of yeast and related fungi, the acentriolar ‘flexible centrosomes’ of flowering plants or the typical centrosomes of mammalian cells consisting of two centrioles embedded in pericentriolar material (review by Kalt and Schliwa, 1993). It is therefore very likely that in addition to having many proteins in common different types of centrosomes will also display unique sets of proteins. Although considerable progress has been made in recent years to unravel the biochemical complexity of centrosomes (reviews: Kalt and Schliwa, 1993; Lange and Gull, 1996), in no system has this goal been achieved so far. In Chlamydomonas reinhardtii several mutants with defects in the structure of the basal apparatus have been characterized (Dutcher and Lux, 1989; Taillon et al., 1992; Ehler et al., 1995; Vashishtha et al, 1996) and γ-tubulin (a typical constituent of MTOCs; Joshi, 1994) was localized to the basal apparatus in interphase cells of C. reinhardtii (Dibbayawan et al., 1995). A different approach towards the identification of basal apparatus proteins depends on the isolation and biochemical characterization of the basal apparatus and its structural components. In this short review we describe the progress in the biochemical identification and (immuno)localization of algal basal apparatus proteins starting with the discovery of the first algal basal apparatus protein, centrin, to the most recent methodological advances involving highly purified basal apparatuses from the green alga Spermatozopsis similis (Geimer et al., 1997a). These studies will ultimately lead to the establishment of a molecular map of the algal basal apparatus.