A C-terminal domain, which prevents secretion of the neuroendocrine protein 7B2 in Saccharomyces cerevisiae, inhibits Kex2 yet is processed by the Yap3 protease

Neuroendocrine secretory protein 7B2: structure, expression and functions

7B2 is an acidic protein residing in the secretory granules of neuroendocrine cells. Its sequence has been elucidated in many phyla and species. It shows high similarity among mammals. A Pro-Pro-Asn-Pro-Cys-Pro polyproline motif is its most conserved feature, being carried by both vertebrate and invertebrate sequences. It is biosynthesized as a precursor protein that is cleaved into an N-terminal fragment and a C-terminal peptide. In neuroendocrine cells, 7B2 functions as a specific chaperone for the proprotein convertase (PC) 2. Through the sequence around its Pro-Pro-Asn-Pro-Cys-Pro motif, it binds to an inactive proPC2 and facilitates its transport from the endoplasmic reticulum to later compartments of the secretory pathway where the zymogen is proteolytically matured and activated. Its C-terminal peptide can inhibit PC2 in vitro and may contribute to keep the enzyme transiently inactive in vivo. The PC2-7B2 model defines a new neuroendocrine paradigm whereby proteolytic activation of prohormones and proneuropeptides in the secretory pathway is spatially and temporally regulated by the dynamics of interactions between converting enzymes and their binding proteins. Interestingly, unlike PC2-null mice, which are viable, 7B2-null mutants die early in life from Cushing's disease due to corticotropin ('ACTH') hypersecretion by the neurointermediate lobe, suggesting a possible involvement of 7B2 in secretory granule formation and in secretion regulation. The mechanism of this regulation is yet to be elucidated. 7B2 has been shown to be a good marker of several neuroendocrine cell dysfunctions in humans. The possibility that anomalies in its structure and expression could be aetiological causes of some of these dysfunctions warrants investigation.

Disruption of the Saccharomyces cerevisiae YAP3 gene reduces the proteolytic degradation of secreted recombinant human albumin

Expression of recombinant human albumin (rHA) in Saccharomyces cerevisiae resulted in secretion of both mature albumin and a 45 kDa degradation product, comprising an N-terminal fragment of rHA with heterogeneous C-termini between residues 403 and 409 (Geisow et al., 1991). Site-directed mutagenesis of the human albumin gene (HA) to change Arg410 to Ala (R410A) caused a significant reduction in the amount of fragment produced. Mutation of the adjacent dibasic site Lys413 Lys414 had little effect in isolation, but in combination with the R410A mutation resulted in a further reduction in the amount of rHA fragment produced. This reduction could be duplicated with nature-identical rHA by disruption of the gene for an aspartyl protease (YAP3), alone or in conjunction with disruption of the KEX2 gene. Disruption of KEX2 alone did not result in any improvement in the degree of degradation of the rHA. Reduced degradation was also observed when an rHA-human growth hormone fusion protein was secreted from a yap3 strain, suggesting that such strains may have a general utility for heterologous protein secretion.

The molecular biology of secreted enzyme production by fungi

Enzymes from filamentous fungi are already widely exploited, but new applications for known enzymes and new enzymic activities continue to be found. In addition, enzymes from less amenable non-fungal sources require heterologous production and fungi are being used as the production hosts. In each case there is a need to improve production and to ensure quality of product. While conventional, mutagenesis-based, strain improvement methods will continue to be applied to enzyme production from filamentous fungi the application of recombinant DNA techniques is beginning to reveal important information on the molecular basis of fungal enzyme production and this knowledge is now being applied both in the laboratory and commercially. We review the current state of knowledge on the molecular basis of enzyme production by filamentous fungi. We focus on transcriptional and post-transcriptional regulation of protein production, the transit of proteins through the secretory pathway and the structure of the proteins produced including glycosylation.

Role of mitochondria and C-terminal membrane anchor of Bcl-2 in Bax induced growth arrest and mortality in Saccharomyces cerevisiae

In mammalian cells, the Bcl-2 and Bcl-x(L) proteins suppress programmed cell death whereas the topographically similar Bax protein accelerates the apoptotic process. Recently published data suggest that expression of the human Bax-alpha gene is lethal for the yeast Saccharomyces cerevisiae and that this toxicity can be overcome by co-expressing Bcl-2 or Bcl-x(L). Our findings corroborate these results. However, we find that although Bax induction invariably stops cell growth under all circumstances, it does not lead to death in 'petite' cells. Petites cannot respire because they lack functional mitochondria. It seems that in 'grande' cells, which do possess normal mitochondrial DNA, nutritional limitation is critical for increased mortality. Surprisingly, murine Bcl-2 lacking the membrane anchor of human Bcl-2 has no effect on grande cells, but can efficiently rescue petites in rich medium. It has been suggested that the C-terminal membrane anchor of human Bcl-2 may have a crucial role in rescuing apoptosis in mammalian cells. When murine Bcl-2 is fused to the membrane anchor of yeast mitochondrial Mas70 protein, the Bcl-2 variant mBcl-2-mma rescues not only petites but also grandes, just like human Bcl-x(L). The rescuing ability of Bcl-x(L), which contains its own membrane anchor, surpasses that of mBcl-2-mma. Our results indicate that the process involving Bax-induced growth inhibition followed by possible lethality, and the rescuing effect of Bcl-2 and Bcl-x(L) is linked to yeast mitochondrial function. We propose a model which is consistent with these observations.