W3 Lysosomes Flashcards
Extracellular substrates: Endocytosis
Fluid-phase endocytosis of molecules (uptake of larger molecules) and lipoproteins (includes receptor-mediated endocytosis)
Fluid-phase endocytosis = no specific binding to membrane post endocytosis + low efficiency
Phagocytosis of particles ≥ 0.5 µm
Formation of lysosome
Selective transport of proteins across the lysosomal membrane
Lysosome mature in early endosome = become more acidic (new enzymes can work) as H+ pumped into lumen of ER
Autophagy (wrapped in ER membrane) wraps around MC = autophagosome
Autophagosome = deliver cytoplasmic components to lysosomes
Receptor-mediated endocytosis
Only substances specific to receptor can enter
Adaptor proteins + clathrin (protein which plays vital role in formation of coated vesicles) triskelions allow budding to form outside of cell
Coated pits invaginate to form a vesicle
pH drops causing dissociation of LDL particle → mature pit cleaved from CM by membrane binding/fission proteins
Clathrin-coated vesicle fuses w/endosome
LDL particle ends up in lysosomes hydrolysed to CL + fatty acid by acid lipase
Phagocytosis
Antigen recognised + particle ingested by phagocyte
Membrane wraps round particle = phagosome
Engulfment facilitated by actin-myosin contractile system
Lysosome + phagosome fusion = phagolysosome
Particle hydrolysed by hydrolytic enzymes inside lysosome
Waste discharged by exocytosis from phagocyte
Lysosomal hydrolases
Only lysosome sugars require (M6P) mannose-6-phosphate
M6P = key targeting signal for acid hydrolase precursor proteins
M6P tag added to proteins in cis-Golgi then moved to trans-Golgi network
M6P tag recognised by M6P receptor protein and binds (pH 6.5-6.7)
Vesicles move M6P-tagged lysosomal enzymes to late endosomes
When pH 6 in late endosome → M6P dissociates from receptor
MPR proteins recycled as they are packed into vesicles (after enzymes reach lysosomes) that bud off the late endosome + return to trans-Golgi network
I cell disease (Mucolipidosis type II) (I for inclusion)
Lysosomal storage disease due to defective phosphotransferase (moves phosphate to mannose) in Golgi
Proteins secreted outside cell due to absence of M6P
Skeletal abnormalities, development delay, enlarged liver/spleen, impaired hearing
Death from pneumonia or congestive heart failure usually occurs within the first decade of life
Autosomal-recessive disorder caused by a deficiency of the enzyme UDP-N -acetylglucosamine: N-acetylglucosamine-1-phosphotransferase (“GlcNAc phosphotransferase”)
Formation of M6P tag in Golgi apparatus
Tag added in cis-golgi apparatus
Reaction between UDP + N-acetylglucosamine
GlcNAc-1-phosphotransferase catalyze adding GlcNAc-1-phosphate to the terminal mannose residue of the oligosaccharides on lysosomal enzymes + catalyzes the N-linked glycosylation of asparagine residues with M6P
N-acetylglucosamine-1-phosphodiester alpha-N-acetylglucosaminidase (encoded by NAGPA)
cleaves off GlcNAc residue
So lysosomal hydrolase w/M6P exposed
Low pH in the endosomal/prelysosomal compartment, the lysosomal enzyme-MPR complex dissociates and then the enzyme is delivered to the lysosome
Why do mutations in lysosomal hydrolyses cause substrate accumulation?
Enzymes in metabolic pathway regulated by one gene through RNA + protein products so failure of metabolic pathway = substrate (precursor molecule) accumulation
Microautophagy
Microautophagy (invagination of the lysosomal membrane) = double membrane vesicles formed
Macroautophagy
Macroautophagy (Cytosol or organelles wrapped in ER membrane, which then fuses with lysosomes)
Mutations in GNPTG
Mutations in GNPTG cause mucolipidosis type III (Pseudo-Hurler polydystrophy)
Mutations in GNPTAB
Mutations in GNPTAB cause mucolipidosis type II (I-cell disease) or type III
