Academic journal article Genetics

Evidence That an Unconventional Actin Can Provide Essential F-Actin Function and That a Surveillance System Monitors F-Actin Integrity in Chlamydomonas

Academic journal article Genetics

Evidence That an Unconventional Actin Can Provide Essential F-Actin Function and That a Surveillance System Monitors F-Actin Integrity in Chlamydomonas

Article excerpt

ACTIN is one of the most highly conserved proteins across the full range of the eukaryotic phylogeny (Figure 1, A and B; Hightower and Meagher 1986; Sheterline et al. 1999); for example, human, yeast, and higher-plant actins are all ~90% identical in sequence. This sequence conservation suggests that actin also has highly conserved and critical functions. Indeed, since the demonstration that actin is essential in nonmotile budding yeast cells (Shortle et al. 1982), it has been widely presumed that it is essential for the survival and function of most, if not all, eukaryotic cells (although this appears to have been demonstrated rigorously in few, if any, other cases). Consistent with this view, actin has been shown to be involved in a diverse range of important processes, including vesicle transport and endocytosis, the determination of cell shape, cell motility, and cytokinesis (Korn 1982; Sheterline et al. 1999; Doherty and McMahon 2008; Balasubramanian et al. 2012). In all of these roles, actin is thought to function not as globularmonomers ("G-actin") but as filaments ("F-actin"), and filament formation is known to be tightly regulated, prominently by actin-binding proteins such as profilin, the formins, and the Arp2/3 complex (Doherty and McMahon 2008).

Many organisms contain multiple genes that encode actins with unknown degrees of actual or potential functional overlap; for example, mammals contain six (Perrin and Ervasti 2010), the slime mold Dictyostelium discoideum contains 33 (Joseph et al. 2008), and the plant Arabidopsis thaliana contains at least 10 (McDowell et al. 1996). Because of the difficulties that the potential redundancy of actin poses for genetic analyses, many studies of actin function have relied instead on polymerizationblocking drugs such as the cytochalasins and the latrunculins LatA and LatB. In particular, LatA and LatB are highly effective in depolymerizing most if not all actins that have been studied to date (Spector et al. 1983, 1989; Ayscough 1998). However, such drug studies have unavoidable ambiguity because of the possibility that not all actin family members are equally sensitive to the drug and the danger of unknown side effects.

Given this background, the unicellular green alga Chlamydomonas reinhardtii has presented an opportunity but also a paradox and a challenge. It has a single gene (IDA5) encoding a conventional actin (Figure 1, A and B), and localization studies have suggested that IDA5 may have multiple roles both during vegetative growth and during mating (Harper et al. 1992; Ehler and Dutcher 1998; Kato-Minoura et al. 1998; Kovar et al. 2001; Avasthi et al. 2014). However, electron microscopy and labeling with fluorescent phallotoxin detected F-actin structures only in fertilization tubules (Detmers et al. 1983; Wilson et al. 1997; Hirono et al. 2003), raising the surprising possibility that IDA5 might function during vegetative growth as G-actin rather than in filaments, as originally suggested by Harper et al. (1992). In another surprise, a screen for mutants defective in swimming behavior and flagellar structure yielded a null mutation in IDA5 (originally ida5, henceforth ida5-1; a frameshiftresulting in a complete loss of IDA5 protein) (Kato et al. 1993; Kato-Minoura et al. 1997). This mutation has essentially no effect on cell proliferation, although it does affect intraflagellar transport, flagellar regeneration, and fertilization-tubule formation (Kato et al. 1993; Kato-Minoura et al. 1997; Avasthi et al. 2014). Moreover, although very high doses of cytochalasin D caused temporary shortening of the flagella (Dentler and Adams 1992), they had no effect on proliferation (Harper et al. 1992).

These observations have suggested that Chlamydomonas cell-shape determination, vesicle transport and endocytosis, and cytokinesis might be completely independent of F-actin function. However, interpretation of this surprising hypothesis has been complicated by the presence in Chlamydomonas of a gene (NAP1) that encodes an unconventional actin (Kato-Minoura et al. …

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