Organelle Structure and Function | A Level Notes
In protein synthesis, the main cellular ..
As in other eukaryotes cells possess an endoplasmic reticulum into which most membrane or secretory proteins are inserted co-translationally. Some lumenal ER-resident proteins are retained in the ER by KDEL or HDEL signals in their extreme C-termini. cells also possess Golgi stacks, the next organelle through which secretory proteins pass. Unlike mammalian cells, invertebrates such as have many small "mini stacks" throughout the cytoplasm of most cells rather than one large stack positioned near the nucleus. From the Golgi, secretory proteins and some membrane proteins proceed to either the plasma membrane or to the endosomal system, depending upon various signals embedded in the primary sequence of the proteins. does not possess any obvious homologs of the mannose-6-phosphate receptors, and so may not use the mannose-6-phosphate system for tagging and sorting newly synthesized lysosomal hydrolases. Worms may instead use amino acid based signals and a vps10/sortillin receptor type system for such sorting, as is known to be the case in yeast.
cellular structure that carries out protein synthesis
One component of the general secretory pathway that has been studied in significant detail in is , a part of the endoplasmic reticulum vesicle coat complex known as COPII (). COPII is known to be the primary vesicle coat complex used in yeast and mammalian cell transport from the ER to the Golgi. Roberts et al. identified a single mutant allele of in a screen for embryonic lethals defective in cuticle synthesis as assayed by a cuticle collagen reporter, (). These authors went on to show severe defects in cuticle synthesis in mutants resulting in accumulation of collagen intracellularly, presumably in the ER. Zygotic embryonic lethality during elongation was found in homozygous mutants derived from heterozygous mothers. Normal progression through early development presumably relies upon maternally derived . RNAi experiments revealed a requirement for during larval development, particularly during molting. Adults depleted of by RNAi also showed severe germline defects including binucleate oocytes, lack of yolk uptake by oocytes resulting from a failure of yolk receptors () to reach the cell surface, and premature maturation/partitioning of individual germ cells, possibly resulting from loss of cell surface receptors in the distal gonad. A partially functional ::GFP reporter gene indicated that is broadly expressed at all life stages, and that in hypodermal cells the protein is concentrated in distinct foci. In the embryonic hypodermis these foci were enriched apically at the periphery of the endoplasmic reticulum. These positive foci likely represent ER exit sites where newly synthesized cargo molecules concentrate and are packaged into COPII coated vesicles for delivery to the Golgi.