microautophagy

Microautophagy is a cellular degradation pathway in which cytosolic components are delivered to lysosomes or vacuoles by direct sequestration through invagination and scission of the lysosomal (or vacuolar) membrane. This results in the formation of intralysosomal vesicles that are broken down inside the lysosome, releasing their constituent building blocks for reuse.

There are two commonly described modes. In lysosomal microautophagy, the lysosomal membrane itself engulfs portions of the cytosol or selected cargo. In endosomal microautophagy, cytosolic materials are delivered to late endosomes or multivesicular bodies via inward budding of the endosomal membrane, after which intralumenal vesicles are degraded following fusion with lysosomes. The precise molecular machinery and regulation can vary among organisms; some forms involve ESCRT complexes and may be independent of certain autophagy-related genes, while others may rely on different sets of core autophagy factors.

Microautophagy contributes to cellular homeostasis by recycling nutrients during starvation, removing damaged or excess cytosolic components, and participating in quality control. It can be non-selective, particularly under nutrient stress, or, in some contexts, show selectivity for particular substrates, including damaged proteins or organelles. The relative contributions of microautophagy versus macroautophagy and chaperone-mediated autophagy depend on cell type, organism, and physiological conditions.

Across eukaryotes, microautophagy operates in lysosomes and plant vacuoles and complements other autophagic pathways. Its study enhances understanding of intracellular quality control, nutrient sensing, and adaptive responses to stress.