Another essential protein thought
to be involved in autophagy is the Microtubule-associated protein 1A/1B-light
chain 3B (MAP LC3B or simply LC3) from the ATG8 protein family. (Shpilka T).
During autophagy ATG genes, ATG5 and ATG7, among others, arrange a cascade of
reactions leading to lipidation of LC3. (Metlagel Z). When induced by nutrient
starvation a cytosolic form of LC3 (LC3-I) is conjugated to
phosphatidylethanolamine to form the LC3-phosphotadylethanolamine conjugate
(LC3-II). At the last step of autophagy, while the autolysosome is formed and
the structures within it degraded, LC3-II in the autosomal lumen is degraded by
lysosomal hydrolases. (Tanida I). The rate at which LC3-II is degraded and
replaced reflects therefore starvation-induced autophagic activity. Degradation
of LC3-II inside the autolysosome is estimated by measuring the amount of LC3-I
(cytosolic form) relative to LC3-II (membrane-bound lipidated form) and
comparison of two samples with and without lysosomal inhibitor treatment (e.g.
NH4Cl, Chloroquine, Baflomycine). (Mizushima, )
Under conditions in which autophagy is not induced, e.g.
nutrient-rich conditions, LC3-positive puncta barely exist. This is why the
appearance of LC3 positive puncta sugguest the induction of autophagy.
(Gniadek, T. J)