LC3II

LC3-II is the lipidated form of the autophagy-related protein LC3, derived from MAP1LC3. It is widely used as a marker of autophagosomes in eukaryotic cells and associates with autophagosomal membranes during autophagy.

Biogenesis and processing of LC3-II begin with the synthesis of pro-LC3, which is cleaved by the protease ATG4 to form LC3-I. LC3-I is then activated by the E1-like enzyme ATG7 and transferred to the E2-like enzyme ATG3, where it is conjugated to phosphatidylethanolamine to generate LC3-II. The ATG12-ATG5-ATG16L1 complex acts as a scaffold that helps specify membrane sites and promote elongation of the autophagosomal membrane.

Functionally, LC3-II decorates the surface of autophagosomes and participates in cargo recruitment through interactions with LC3-interacting region (LIR) motifs in adaptor proteins such as p62/SQSTM1. LC3-II thus plays a role in both the formation of autophagosomes and the selective capture of cytosolic cargo. During maturation, LC3-II can be recycled back to LC3-I by ATG4 after the autophagosome fuses with lysosomes.

Detection and interpretation of LC3-II levels are common in autophagy studies. On Western blots, LC3-II runs as a lower molecular weight band than LC3-I, and the ratio of LC3-II to LC3-I or the absolute amount of LC3-II is used to infer autophagosome abundance. However, LC3-II accumulation can result from increased formation or impaired degradation, so flux measurements often require lysosomal inhibitors (e.g., chloroquine or bafilomycin A1) and complementary markers such as p62 degradation for accurate interpretation.

There are several LC3 isoforms (LC3A, LC3B, LC3C), with LC3B commonly used as a standard in research.