A broad spectrum of actin paralogs inParamecium tetraureliacells display differential localization and function

Abstract

To localize the different actin paralogs found in Paramecium and to disclose functional implications, we used overexpression of GFP-fusion proteins and antibody labeling, as well as gene silencing. Several isoforms are associated with food vacuoles of different stages. GFP-actin either forms a tail at the lee side of the organelle, or it is vesicle bound in a homogenous or in a speckled arrangement, thus reflecting an actin-based mosaic of the phagosome surface appropriate for association and/or dissociation of other vesicles upon travel through the cell. Several paralogs occur in cilia. A set of actins is found in the cell cortex where actin outlines the regular surface pattern. Labeling of defined structures of the oral cavity is due to other types of actin, whereas yet more types are distributed in a pattern suggesting association with the numerous Golgi fields. A substantial fraction of actins is associated with cytoskeletal elements that are known to be composed of other proteins. Silencing of the respective actin genes or gene subfamilies entails inhibitory effects on organelles compatible with localization studies. Knock down of the actin found in the cleavage furrow abolishes cell division, whereas silencing of other actin genes alters vitality, cell shape and swimming behavior.

Identification, Localization, and Functional Implications of the Microdomain-Forming Stomatin Family in the Ciliated Protozoan Paramecium tetraurelia

“…6 and 7), we know that the subplasmalemmal space, though very narrow, contains actin (41). This is also true of the surface of some stages of the food vacuoles (88), in agreement with their decoration by Stomatin in the current paper, although we do not yet know whether this occurs in equivalent cyclosis stages. Of particular interest would be any correlation with acidification which starts a short time after pinch-off (35).…”

“…Stomatin may serve as a scaffold for the assembly of unknown mechanosensors in this case, and actin coating may serve to protect the organelle membrane. In fact, mechanical stress as an inducer of local actin polymerization is well established, at least for the cell surface (86,87), and actin covers phagosomes in Paramecium (88).…”

Novel Types of Ca2+ Release Channels Participate in the Secretory Cycle of Paramecium Cells

“…1 and 13). A similarly broad diversification has been observed also within many other Paramecium gene families (32,84,86,101), which can be explained as follows. Owing to a recent whole-genome duplication, Paramecium genes often occur as pairs of closely related paralogues (5), which is the case with 20 genes of the 34 CRCs described here.…”

“…In a first approach, we examined whether silencing of IP 3 R N or CRC-IV-1 receptor families has an effect on Paramecium cell division. In general, a slightly reduced division rate during the first 24 h of silencing could be observed, probably due to a lag effect after transfer from normal medium to feeding solution (86). Furthermore, Ca 2ϩ concentration in the culture medium affects division rates of silenced cells.…”

Autophagy in protists

“…Paralogue expansion of other Paramecium gene families has been reported previously, and it appears that genome architecture and gene content in Paramecium have been shaped by multiple rounds of whole genome duplicaton. 64 In instances where examples of unexpected paralogue expansion in Paramecium have been subject to experimental examination (e.g., actin-related proteins 65 and vacuolar ATPase 66 ), the different gene products have often been observed to have distinct cellular functions. This suggests that future study of Paramecium ATG4 and ATG8 paralogues will be of merit.…”