The formation of disulphide bonds is essential to the structure and function of proteins. These bonds rapidly form either cotranslationally or immediately post-translationally in the lumen of the endoplasmic reticulum. Native disulphide pairing for such proteins has been achieved in vitro; however, the rates of reassembly are slow and the conditions non-physiological. To account for these observations, Anfinsen et al. proposed that a 'disulphide interchange protein' was the in vivo catalyst of disulphide bond rearrangement. Other groups discovered an activity with similar characteristics that catalysed the reductive cleavage of insulin and may be associated with insulin degradation, although this result has been disputed. The enzyme involved, protein disulphide isomerase (PDI; EC 5.3.4.1), may be the in vivo catalyst of disulphide bond formation. Here we describe the sequence of cloned rat liver PDI complementary DNA which predicts a protein with two distinct regions homologous with Escherichia coli thioredoxin, a known cofactor in oxidation-reduction reactions. Each of these regions contains the presumed active site sequence Trp-Cys-Gly-His-Cys-Lys, suggesting that PDI, similar in action to thioredoxin, catalyses disulphide bond interchange via an internal disulphide-sulphydryl interchange. The cDNA predicts a signal peptide consistent with the view that PDI is a luminal endoplasmic reticulum protein. PDI messenger RNA, although ubiquitous, is more highly concentrated in secretory cells.

The primary structure of human insulin-like growth factor II receptor, predicted from the complementary DNA sequence, reveals a transmembrane receptor molecule with a large extracellular domain made up of fifteen repeat sequences and a small region homologous to the collagen-binding domain of fibronectin. The structural and biochemical features of the IGF-II receptor appear identical to those of the cation-independent mannose-6-phosphate receptor.

Cryptococcus neoformans is a major opportunistic fungal pathogen in AIDS and other immunosuppressed patients. We have shown that wild-type haploid C. neoformans can develop an extensive hyphal phase under appropriate conditions. Hyphae produced under these conditions are monokaryotic, possess unfused clamp connections, and develop basidia with viable basidiospores. The ability to undergo this transition is determined by the presence of the a-mating type locus and is independent of serotype. The association of the hyphal phase with the a-mating type may explain the preponderance of this mating type in the environment and the nature of the infectious propagule of C. neoformans.Since the discovery of the sexual or perfect state of the human fungal pathogen Cryptococcus neoformans by Kwon-Chung (1, 2), it has been considered a bipolar heterothallic basidiomycete with two mating types, a and a (MA Ta and MA Ta The ability of hyphae to form fruiting body-like structures in the vegetative phase is called monokaryotic, homokaryotic, or haploid fruiting (13). Although haploid fruiting is common in the higher basidiomycetes (13-15), it has not been characterized in C. neoformans, and association with mating type has never been described. Recent observations made in this laboratory and a reassessment of past studies (5, 10, 16) allow us to propose a new hypothesis for the biased mating type ratios and the nature of the infectious propagule of C. neoformans. We show here that a strains of C. neoformans can undergo a true dimorphic transition from a haploid yeast phase to a hyphal phase from which vegetative growth can continue indefinitely. The hyphal phase is induced by nitrogen starvation on a solid surface. Basidia bearing viable basidiospores are also produced, all of which are a in mating type. The ability of a but not a cells to form basidiospores under these conditions provides an attractive explanation for both the mating type bias and the nature of the infectious propagule of C. neoformans. MATERIALS AND METHODSStrains. The strains used in this study are listed in Table 1. Auxotrophic strains were derived from JEC20 (MA Ta) and JEC21 (MA Ta). JEC20 and JEC21 are a well-characterized, haploid, congenic pair of prototrophic laboratory strains that presumably differ only at the mating type locus (17).Media 7327The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Pneumocystis carinii pneumonia is the most common opportunistic infection in AIDS, and accounts for significant morbidity and mortality in these and other immunocompromised patients. P. carinii is a eukaryotic microorganism of uncertain taxonomy that can infect numerous mammalian hosts. Developing from a small, unicellular 'trophozoite' into a 'cyst' containing eight 'sporozoites', its life cycle superficially resembles those seen both in the Protozoa and Fungi. Morphological and ultrastructural observations have lead some investigators to conclude that the organism is a protozoan, while others have felt that it more closely resembles a fungus. Phylogenetic relationships can be inferred from comparisons of macromolecular sequences. Small subunit ribosomal RNAs (16S-like rRNAs) are well-suited for this purpose because they have the same function in all organisms and contain sufficient information to estimate both close and distant evolutionary relationships. Phylogenetic frameworks based upon such comparisons reveal that the plant, animal and fungal lineages are distinct from the diverse spectrum of protozoan lineages. In this letter, phylogenetic analysis of Pneumocystis 16S-like rRNA demonstrates it to be a member of the Fungi.

A cDNA encoding Cryptococcus neoformans orotidine monophosphate pyrophosphorylase (OMPPase) has been isolated by complementation of the cognate Escherichia coli pyrE mutant. The cDNA was used as a probe to isolate a genomic DNA fragment encoding the OMPPase gene (URA5). By using electroporation for the introduction of plasmid DNA containing the URA5 gene, C. neoformans ura5 mutants could be transformed at low efficiency. Ura+ transformants obtained with supercoiled plasmids containing the URA5 gene showed marked mitotic instability and contained extrachromosomal URA5 sequences, suggesting limited ability to replicate within C. neoformans. Transformants obtained with linear DNA were of two classes: stable transformants with integrated URA5 sequences, and unstable transformants with extrachromosomal URA5 sequences.