, a mutation in the Kit protein:A separate mutation, called (Ws) knocksout the , a tyrosine kinase transmembrane receptor,which is produced by the c-kit gene. The mutation is a 12-basedeletion in the c-kit gene (Tsujimura 1991). The kitprotein has a wide variety of functions! Kit is involved in thedevelopment of blood stem cells (precursors to red and white bloodcells), melanoblasts, and primordial germ cells, and melanoblastmigration (Horie 1991). So knocking out the kit proteinwill have a variety of effects, including: depigmentation of certainareas (up to and including entirely white with black eyes), andsometimes anemia, a deficiency of mast cells (and thereforedeficiencies in histamine and serotonin), reproductive problems anddeafness (Kitamura 1994, Hoshino 2000, Sugimoto1995).
Once the pigments are made, they have to be transported to thehair. The pigment particles, called granules, are synthesized inlittle vesicles called melanosomes, which are transported via thedendrites of the melanocyte to the shaft of the growing hair. Thetransport is actually really neat... melanosomes are carried bylittle molecular feet (myosin 5, an actin dependent motor protein)along little branching molecular "ladders" (actin filaments)contained in projections off the melanocyte (dendrites) to the cell'sedge. If this transportation system is affected, transport to thecell edge isn't normal.
Cell Components and Functions of Cell Organelles
Early/Medial Golgi cisternae can be visualized by GFP-tagged AMAN-2 (also called MANS/mannosidase) and RER-1 (a homolog of the yeast retrieval receptor for ER membrane proteins) (). GFP-tagged SYN-16 (a syntaxin 16 homolog) and RAB-6.2 mark late-Golgi compartments (; ; ; ; ). Most of early Golgi compartments appear directly juxtaposed to late Golgi compartments, indicating that Golgi mini-stacks contain early and late compartments in , which is similar to mammals but distinct from budding yeast where early and late Golgi compartments are spatially separated (). Depending upon their sorting in the TGN, newly synthesized proteins are transported to the plasma membrane or endosomes. Lysosomal hydrolases are typically sorted in the TGN for delivery to endosomes by mannose-6-phosphate receptors or Vps10/sortillin type receptors in other organisms. However, does not have any obvious homologs of mannose-6-phosphate receptors or Vps10/sortillin type receptors, leaving it a mystery for the moment as to how the sorting of lysosomal hydrolases is achieved. Interestingly, Golgi compartments are often observed closely juxtaposed to RAB-5-positive endosomes and partly to RME-1 positive tubulo-vesicular recycling endosomes, consistent with the existence of transport pathways linking these organelles (; ; ).
Protein Synthesis in the Cell and the Central Dogma ..
Cell surface membrane proteins, and extracellular macromolecules that bind to them, are internalized by endocytosis, either through cell-surface clathrin-coated pits or through a variety of poorly understood clathrin-independent mechanisms (; ). Internalized cargo is transported to endosomes from which it can be sorted to lysosomes for degradation, recycled to the plasma membrane, often via a distinct recycling endosome compartment, or recycled to the TGN via retrograde recycling (endosome to Golgi transport) (; ). Very large particles, such as whole apoptotic cells, can be internalized by phagocytosis (also called engulfment). Phagosomes interact sequentially with endosomes and lysosomes to form phagolysosomes, which degrade their contents (). Large cytoplasmic organelles and macromolecules can reach the lysosome for degradation via autophagy, a process by which cytoplasmic cargo is encircled by assembly of a distinct double membrane, then fusion of the autophagosome with the lysosome (; see WormBook chapter ).
Organelles Involved in Protein Synthesis ..
Membrane trafficking mediates transport of proteins and lipids within the endomembrane system, which is composed of small vesicles and larger intracellular organelles, including the endoplasmic reticulum (ER), the Golgi complex, endosomes, lysosomes, and autophagosomes () (; ). Protein transport between compartments is mediated in part by budding and fusion of small transport vesicles or tubules, fusion and fission of large organelles, and maturation processes that change organelle identity over time. Many aspects of these processes are regulated by small GTPases belonging to Arf/Sar and Rab families () (). Arf/ Sar GTPases typically regulate the assembly of coat proteins on donor membranes, and are required for the formation of budding vesicles. Rab proteins most commonly regulate the later steps in vesicle transport including motor-driven vesicle movement and vesicle tethering to target membranes. The genome contains 11 genes encoding Arf or Arf-like proteins, 1 gene encoding Sar1, and 31 genes encoding Rab or Rab-like proteins (, see ) (; ). The final step of vesicle transport, membrane fusion, is mediated by soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) (). The SNAREs are classified functionally into v-SNAREs (also called R-SNAREs), located on the vesicles/transport intermediates, and t-SNAREs (also called Q-SNAREs), located on the target membrane. The genome encodes 10 t-SNARE syntaxin homologs, 3 SNAP-25 family proteins (t-SNAREs) and 16 other SNAREs (, ) ().