Actin of Histriculus cavicola: Characteristics of the Highly Divergent Hypotrich Ciliate Actins¹

Abstract

A macronuclear gene-sized molecule carrying an actin gene from the hypotrich ciliate, Histriculus cavicola, was characterized. Southern blot analysis using a coding region probe suggested that actin in H. cavicola is encoded by a single gene. A comparison of the promoter regions indicated that the H. cavicola actin gene has a TATA box in the 5' flanking region in a position identical to those in other oxytrich ciliates. The coding sequence of this gene is not interrupted by any introns, and codes for a protein of 375 amino acid residues. This protein shares a high degree of similarity with other oxytrichid actins, and a relatively low similarity with actins from other eukaryotes. Comparative analyses of sequences indicated that most of the amino acid substitutions in hypotrich actins are found in surface loops, while the core structures are well-conserved. The sites that interact with DNase I and several regions involved in actin-actin contact have diverged considerably in hypotrich actins, while nucleotide-binding sites are the best-conserved interaction motif.

Highly Divergent Actins from Karyorelictean, Heterotrich, and Litostome Ciliates1

“…Out of the 30 residues, 14 were located in subdomain 4, often on the surface of the protein. The surface localization of residues with lowered evolutionary constraints was also reported by Pérez‐Romero et al (1999). Here, we further narrowed down the distribution of those residues onto the surfaces of domains 2–4 (data not shown).…”

“…Actins from some ciliate species such as T. pyriformis (Hirono et al 1989), P. tetraurelia (Díaz‐Ramos et al 1998) and H. caviola (Pérez‐Romero et al 1999) have been known to be unconventional in term of their biochemical and/or sequence properties. In this study, we sequenced actin genes from five more ciliate species B. japonicum, B. musculus, D. nasutum, D. margaritifer, L. striatus , and analyzed them.…”

“…Even though actin genes from ciliates have been sequenced only in a few species, they are found to be highly divergent. For example, Tetrahymena actin (Cupples and Pearlman 1986; Hirono et al 1989) shows only 75% identity with actin from other organisms, Paramecium actin (Díaz‐Ramos et al 1998) shares only 58–77% identity with a broad phyletic spectrum of actins, and Spirotrich ciliate actins (Pérez‐Romero et al 1999) show no more than 65–70% identity to other actins. The phylogenetic analysis of eukaryotic actins (Bhattacharya and Weber 1997) showed that ciliate actin sequences ( Euplotes crassus, Oxytricha fallax ) did not group either with themselves or with the Apicomplexans.…”

Immunolocalization of Actin in Paramecium Cells

“…In previous times, mainly before molecular biology approaches could be undertaken, biochemical, functional, and immunolocalization studies were tried to probe the potential function of F-actin in ciliates such as Paramecium (Tiggemann and Plattner 1981;Cohen et al 1984;Fok et al 1985;Kersken et al 1986a,b), Tetrahymena (Mitchell and Zimmerman 1985;Hirono et al 1987bHirono et al ,1989Hoey and Gavin 1992), Pseudomicrothorax (Hauser et al 1980), Histriculus (Pérez-Romero et al 1999), Climacostomum (Fahrni 1992), and Spirostomum (Zackroff and Hufnagel 1998). However, with ciliates, F-actin-disrupting drugs frequently had to be used in conspicuously high concentrations to abolish, e.g., phagocytosis (Fok et al 1987;Hufnagel 1998,2002).…”

Different stop codon usage in two pseudohypotrich ciliates

“…Therefore, we can tentatively conclude that actin II seems to have had a higher mutation rate. In fact, the two in‐frame TAG codons of actin II are located in high‐rate mutation regions or ‘hot spots’ of ciliate actins (subdomains 2 and 4, respectively: see [6]). The results obtained in E. vannus neither support nor contradict the above conclusion since the two available actin sequences are free of TGA cysteine‐encoding codons, in good agreement with what has been described in E. crassus actin.…”