Cytoplasmic IF proteins were identified in epithelial cells of the gastropod mollusk Helix pomatia and muscle of the nematode Ascaris lumbricoide, based on in vitro filament assembly, sequence principles and sequence

Intermediate filaments (IFs) are a major protein component of the detergent-insoluble cytoskeleton. In vertebrates, a growing multigene family encodes these proteins, which have thus far been divided into six subclasses based on protein and DNA structural homologies (Lendahl et al., 1990; reviewed by Steinert and Roop, 1988). Cytoplasmic IFs (classes I-IV, VI) form a 10 nm filamentous network which extends from the plasma membrane to the nuclear envelope. Their expression patterns are celland tissue-specific while the nuclear IFs or lamins (class V) are expressed by all vertebrate cells (reviewed by Krohne and Benavente, 1986). These proteins do not form an analogous 10 nm network but a meshwork of filaments comprising the karyoskeleton. All IF proteins share a conserved secondary structure consisting of a central alpha-helical rod domain flanked by variable aminoand carboxyl-terminal end domains. The detergent-insoluble cytoskeleton is less well characterized in invertebrates than in vertebrates and most IF classification and study has been done in vertebrate systems. A few invertebrate IF proteins have been identified by cDNA or gene cloning (Dodemont et al., 1990; Szaro et al., 1991; Gruenbaum et al., 1988) or by protein sequencing and filament assembly (Weber et al., 1988, 1989; Bartnik et al., 1985, 1986). All have been classified as Class V IFs regardless of their tissue origin, implying less IF complexity in invertebrates. Cytoplasmic IF proteins were identified in epithelial cells of the gastropod mollusk Helix pomatia and muscle of the nematode Ascaris lumbricoide, based on in vitro filament assembly, sequence principles and sequence identities (Bartnik et al., 1985, 1986; Weber et al. 1988, 1989). Invertebrate IF genes characterized include the nuclear lamin gene, Dm0, from Drosophila melanogaster (Osman et al., 1990) and a lamin-related IF gene in Helix aspersa (Dodemont et al., 1990), which encodes a cytoplasmic IF. cDNA sequence recently reported for two neurofilament proteins of the squid Loligo pealei also revealed class V IF characteristics (Szaro et al., 1991). Typical IF morphology was absent in an electron microscope survey of arthropods (Bartnik and Weber, 1989), leading to the suggestion that cytoplasmic IFs may not be present in this group. However, a group of IF-related proteins have been implicated in Drosophila by morphological and immunological criteria (Falkner et al., 1981; Walter and Biessmann, 1263 Journal of Cell Science 104, 1263-1272 (1993) Printed in Great Britain © The Company of Biologists Limited 1993