They are found in animals and in unicellular organisms including
Apicomplexa protozoans. They are also found in
Entamoeba. Dense granules play a major role in
Toxoplasma gondii. When the parasite invades it releases its dense granules which help to create the
parasitophorous vacuole.
Toxoplasma gondii T. gondii contains organelles called unique organelles including dense granules. Dense granules, along with other secretory vesicles such as a
microneme and
rhoptry secrete proteins involved in the gliding motility, invasion, and parasitophorous vacuole formation of Toxoplasma gondii. Dense granules specifically secrete their contents several minutes after parasite invasion and localization into the parasitophorous vacuole. Proteins released from these specialized organelles are critical to adapting to the intracellular environment of the invaded host cell and contribute to parasitophorous vacuolar structure and maintenance.
Structure and Biogenesis Dense granules in
T. gondii are spherical, electron dense bodies that resemble secretory
vesicles in mammalian cells about 200 nm in diameter and most likely form from budding off the trans-golgi network. Dense granule protein aggregation and retention is vital to maintaining dense granule biogenesis. This process is thought to follow the sorting-by-retention model in higher eukaryotes due to the morphological similarities of
T. gondii's dense granule and higher eukaryotes' dense core granules. The proposition includes the accumulation of secretory proteins within the granules that prevents their escape from maturing dense granules in the trans-golgi network by constitutive vesicles budding. Additionally, dense granule formation follows a
clathrin dependent matter at the trans-golgi network.
T. gondii expresses one clathrin heavy chain (CHC1) important for forming micronemes and rhoptries in T. gondii, but the adaptor important for targeting CHC1 to dense granules remains unknown. After clathrin is recruited, the mature dense granules bud off the golgi apparatus and are shuttled to plasma membrane release sites in order to secrete their contents.
Trafficking and Secretion of Dense Granule Proteins (GRAs) The majority of GRA proteins contain an N-terminal ER-targeting signal peptide and enter the secretory pathway via synthesis and translocation at the
rough endoplasmic reticulum, and the signal sequence is thought to be cleaved off though not proven. Many GRA proteins contain a single transmembrane domain, meaning that the proteins are translocated across the endoplasmic reticulum lumen, exported to and shuttled through the
golgi apparatus, and eventually secreted from the parasite into the vacuolar space or parasitophorous vacuolar membrane.
SNARE protein complexes drive the transport and docking of vesicles with proteins from the endoplasmic reticulum to the golgi body and vice versa. Once the dense granule organelle is fully matured, the organelle appears to directly fuse with the
plasma membrane between gaps of the parasite's inner membrane complex.
Functions According to the stage of infection, the number of dense granules present in a parasite may vary from approximately 15 in tachyzoites and sporozoites, 8–10 in bradyzoites and 3–6 in merozoites. For nutrient acquisition, GRA proteins have been implicated in scavenging host lipids, such as sphingolipids from host Golgi derived vesicles, cholesterol from host endosomal or lysosomal vesicles, and other lipids from host neutral lipid droplets. GRA proteins also are involved in host protein uptake into the parasite. Further research is needed to characterize the function and prevalence of all GRA proteins. ==In multicellular organisms==