Out with a bang! Tetrahymena as a model system to study secretory granule biogenesis

Genomic and Proteomic Evidence for a Second Family of Dense Core Granule Cargo Proteins in Tetrahymena thermophila

In addition to a family of structurally related proteins encoded by the Granule lattice (GRL) genes, the dense core granules in Tetrahymena thermophila contain a second, more heterogeneous family of proteins that can be defined by the presence of a domain homologous to beta/gamma-crystallins. The founding members of the family, Induced during Granule Regeneration 1 (IGR1) and Granule Tip 1 (GRT1), were identified in previous screens for granule components. Analysis of the recently sequenced T. thermophila macronuclear genome has now uncovered 11 additional related genes. All family members have a single beta/gamma-crystallin domain, but the overall predicted organization of family members is highly variable, and includes three other motifs that are conserved between subsets of family members. To demonstrate that these proteins are present within granules, polypeptides from a subcellular fraction enriched in granules were analyzed by mass spectrometry. This positively identified four of the predicted novel beta/gamma-crystallin domain proteins. Both the functional evidence for IGR1 and GRT1 and the variability in the overall structure of this new protein family suggest that its members play roles that are distinct from those of the GRL family.

Molecular crowding as a mechanism for tick secretory granule biogenesis

During feeding ticks secrete bioactive components into the host to counter-act its immune and hemostatic defense systems. These bioactive components are stored in secretory granules that are secreted during feeding in an exocrine stimulus-response type of mechanism. All proteins destined for secretion are packaged into these granules during granule biogenesis. Up to date no mechanism for granule biogenesis has been proposed, except to note that biogenesis occurs under conditions of high protein and calcium concentrations in an acidic environment. Previously, the most abundant proteins (TSGPs) found in the salivary glands of the soft tick, Ornithodoros savignyi, were suggested to play a part in granule biogenesis, based on their high abundance. The TSGPs are part of the lipocalin family, of which numerous members have been identified in ticks. We consider here the high concentrations of the TSGPs in salivary glands and what effect this will have on the crowded environment inside the secretory granules. It is shown that the TSGPs occur at concentrations that will lead to molecular crowding of which one result is the non-specific aggregation of components to reduce crowding effects. Aggregation of proteins as a mechanism of granule biogenesis has been proposed before, but not in terms of molecular crowding. We thus propose molecular crowding as the general mechanism of granule biogenesis, in tick secretory granules, but can also be extended to other forms of secretory granules in general.

Location, Location, Location: Trafficking and Function of Secreted Proteases of Toxoplasma and Plasmodium

The Apicomplexan parasites Toxoplasma gondii and Plasmodium species are obligate intracellular parasites that rely upon unique secretory organelles for invasion and other specialized functions. Data is emerging that proteases are critical for the biogenesis of micronemes and rhoptries, regulated secretory organelles reminiscent of dense core granules and secretory lysosomes of higher eukaryotes. Proteases targeted to the Plasmodium food vacuole, a unique organelle dedicated to hemoglobin degradation, are also critical to parasite survival. Thus study of the targeting and function of the proteases of the Apicomplexa provides a fascinating model system to understand regulated secretion and secretory organelle biogenesis.