Cell-cell Response Research Articles

Glycolipid intermembrane transfer is accelerated by HET-C2, a filamentous fungus gene product involved in the cell-cell incompatibility response.

Among filamentous fungi capable of mycelial growth, het genes play crucial roles by regulating heterokaryon formation between different individuals. When fusion occurs between fungal mycelia that differ genetically at their het loci, the resulting heterokaryotic cells are quickly destroyed. It is unclear how het gene products of Podospora anserina trigger heterokaryon incompatibility. One unexplored possibility is that glycosphingolipids play a role because the het-c2 gene encodes a protein that displays 32% sequence identity and an additional 30% similarity to the mammalian glycolipid transfer protein. Here, P. anserina protoplasts containing wild-type het-c2 genes were shown to have greater glycosphingolipid transfer activity than protoplasts with disrupted het-c2 genes, a condition previously linked to altered cell compatibility following hyphal fusion. The observed glycolipid transfer activity could not be accounted for by nonspecific lipid transfer protein activity. Direct assessment showed that purified, recombinant HET-C2 accelerates the intermembrane transfer of glycolipid in vitro, but that the HET-C2 activity is mitigated much less by negatively charged membranes than the mammalian glycolipid transfer protein. The findings are discussed within the context of HET-C2 being a member of an emerging family of ancestral sphingolipid transfer proteins that play important roles in cell proliferation and accelerated death.

Conservation of a receptor/signal transduction system

The budding yeast Saccharomyces cerevisiae is a singlecell organism whose process of mating provides an excellent opportunity to study cell-cell interactions and response to diffusible factors. In this process, two partners of opposite type (a and a) fuse to produce a diploid cell. Each partner produces a peptide mating factor (a-factor or a-factor) that acts on the opposite cell type to prepare cells for mating by inducing expression of various genes whose products are necessary for cell and nuclear fusion (Tiueheart et al., MCB 7, 2316-2328, 1987; Rose et al., MCB 6, 3490~3497,1986) and by causing arrest in the Gl phase of the cell cycle. How do these mating factors trigger the multitude of cellular responses? The surprising answer, as the outlines of a molecular understanding become visible, seems to be that the machinery that yeast uses to respond to mating factors has similarity to the apparatus used in many other cells, including cells of the nervous system, for various signal transductions. The Receptors The receptor for yeast a-factor is coded by the STE2 gene, and the receptor for a-factor by the STE3 gene (Burkholder and Hartwell, NAR 73,8463-84751985; Nakayama et al., EMBO J. 4, 2643-2648, 1985; Hagen et al., PNAS 83, 1418-1422, 1986). The deduced amino acid sequence of the STf2 and STE3 products revealed that they are members of the rhodopsinlp-adrenergic receptor/muscarinic acetylcholine receptor family of integral membrane proteins. All of these proteins contain seven hydrophobic segments, each long enough to span a lipid bilayer membrane (see figure). These receptors are hypothesized to exhibit a transmembrane arrangement similar to that of bacteriorhodopsin, for which a partial three-dimensional structure is available. Amino acid sequence similarity among these receptors is modest; similarity between STf2 and STf3 polypeptides is virtually absent. The yeast receptors thus provide a striking example of gene products of similar function in which the topology of the protein (its arrangement with respect to the membrane), not primary sequence, has been strongly conserved (figure). The ligands for this receptor family are diverse: a skinny lipid chromophore (retinal) and small organic molecules (epinephrine and acetylcholine). The ligands for STE2 and STE3 proteins, a-factor and a-factor, respectively, are peptides of 13 and 12 amino acids (a-factor is probably a modified peptide). Intracellular Machinery Receptor switching experiments suggest that the response pathway in a and a cells may differ only in the type of receptor and that the intracellular machinery involved in subsequent transmission of the signal evoked by binding of the mating factors is the same in both cell types. It Minireview