Plant defensins are a family of small Cys-rich antifungal proteins that play important roles in plant defense against invading fungi. Structures of several plant defensins share a Cys-stabilized a/b-motif. Structural determinants in plant defensins that govern their antifungal activity and the mechanisms by which they inhibit fungal growth remain unclear. Alfalfa (Medicago sativa) seed defensin, MsDef1, strongly inhibits the growth of Fusarium graminearum in vitro, and its antifungal activity is markedly reduced in the presence of Ca 21. By contrast, MtDef2 from Medicago truncatula, which shares 65% amino acid sequence identity with MsDef1, lacks antifungal activity against F. graminearum. Characterization of the in vitro antifungal activity of the chimeras containing portions of the MsDef1 and MtDef2 proteins shows that the major determinants of antifungal activity reside in the carboxy-terminal region (amino acids 31-45) of MsDef1. We further define the active site by demonstrating that the Arg at position 38 of MsDef1 is critical for its antifungal activity. Furthermore, we have found for the first time, to our knowledge, that MsDef1 blocks the mammalian L-type Ca 21 channel in a manner akin to a virally encoded and structurally unrelated antifungal toxin KP4 from Ustilago maydis, whereas structurally similar MtDef2 and the radish (Raphanus sativus) seed defensin Rs-AFP2 fail to block the L-type Ca 21 channel. From these results, we speculate that the two unrelated antifungal proteins, KP4 and MsDef1, have evolutionarily converged upon the same molecular target, whereas the two structurally related antifungal plant defensins, MtDef2 and Rs-AFP2, have diverged to attack different targets in fungi.

The proper localization of resident membrane proteins to the trans-Golgi network (TGN) involves mechanisms for both TGN retention and retrieval from post-TGN compartments. In this study we report identification of a new gene, GRD20, involved in protein sorting in the TGN/endosomal system of Saccharomyces cerevisiae. A strain carrying a transposon insertion allele of GRD20 exhibited rapid vacuolar degradation of the resident TGN endoprotease Kex2p and aberrantly secreted approximately 50% of the soluble vacuolar hydrolase carboxypeptidase Y. The Kex2p mislocalization and carboxypeptidase Y missorting phenotypes were exhibited rapidly after loss of Grd20p function in grd20 temperature-sensitive mutant strains, indicating that Grd20p plays a direct role in these processes. Surprisingly, little if any vacuolar degradation was observed for the TGN membrane proteins A-ALP and Vps10p, underscoring a difference in trafficking patterns for these proteins compared with that of Kex2p. A grd20 null mutant strain exhibited extremely slow growth and a defect in polarization of the actin cytoskeleton, and these two phenotypes were invariably linked in a collection of randomly mutagenized grd20 alleles. GRD20 encodes a hydrophilic protein that partially associates with the TGN. The discovery of GRD20 suggests a link between the cytoskeleton and function of the yeast TGN.

The yeast COG complex has been proposed to function as a vesicle-tethering complex on an early Golgi compartment, but its role is not fully understood. COG complex mutants exhibit a dramatic reduction in Golgi-specific glycosylation and other defects. Here we show that a strain carrying a COG3 temperature-sensitive allele,