Microautophagy: definition, classification, and the complexity of the underlying mechanisms

Recently, rapid progress in the field of microautophagy (MI-autophagy) revealed the existence of multiple subtypes that differ in both intracellular membrane dynamics and molecular mechanisms. As a result, a single umbrella term "microautophagy" has become too vague, even creating some confusion among researchers both within and outside the field. We herein describe different subtypes of MI-autophagic processes and propose a systematic approach for naming them more accurately.Abbreviation: ATG, autophagy related; e-MI, endosomal microautophagy; ER, endoplasmic reticulum; ESCRT, endosomal sorting complex required for transport; EV, extracellular vesicle; HSPA8/HSC70, heat shock protein family A (Hsp70) member 8; ILVs, intralumenal vesicles; l-MI, lysosomal microautophagy; MAP1LC3/LC3, microtubule associated protein 1 light chain 3; MCOLN1, mucolipin TRP cation channel 1; microautophagy, MI-autophagy; MVBs, multivesicular bodies; SQSTM1, sequestosome 1; v-MI, vacuolar microautophagy.