Viruses, despite their simple structural composition, engage in intricate and complex interactions with their hosts due to their parasitic nature. A notable demonstration of viral behavior lies in their exploitation of lysosomes, specialized organelles responsible for the breakdown of biomolecules and clearance of foreign substances, to bolster their own replication. The man-nose-6-phosphate (M6P) pathway, crucial for facilitating the proper transport of hydrolases into lysosomes and promoting lysosome maturation, is frequently exploited for viral manipulation in support of replication. Recently, the discovery of lysosomal enzyme trafficking factor (LYSET) as a pivotal regulator within the lysosomal M6P pathway has introduced a fresh perspective on the intricate interplay between viral entry and host factors. This groundbreaking revelation illuminates unexplored dimensions of these interactions. In this review, we endeavor to provide a thorough overview of the M6P pathway and its intricate interplay with viral factors during infection. By consolidating the current understanding in this field, our objective is to establish a valuable reference for the development of antiviral drugs that selectively target the M6P pathway.

Vertebrate cells rely on mannose-6-phosphate (M6P) modifications to deliver most lumenal hydrolases to the lysosome. As a critical trafficking signal for lysosomal enzymes, the M6P biosynthetic pathway has been thoroughly investigated. However, its regulatory mechanism is largely unknown. Here, we summarize three recent studies that independently discovered LYSET/TMEM251/GCAF as a key regulator of the M6P pathway. LYSET/TMEM251 directly interacts with GNPT, the enzyme that catalyzes the transfer of M6P, and is critical for its activity and stability. Deleting LYSET/TMEM251 impairs the GNPT function and M6P modifications. Consequently, lysosomal enzymes are mistargeted for secretion. Defective lysosomes fail to degrade cargoes such as endocytic vesicles and autophagosomes, leading to a newly identified lysosomal storage disease in humans. These discoveries open up a new direction in the regulation of the M6P biosynthetic pathway.Abbreviations: ER: endoplasmic reticulum; GNPT: GlcNAc-1-phosphotransferase; KO: knockout; LMP: lysosome membrane protein; LYSET: lysosomal enzyme trafficking factor; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; M6P: mannose-6-phosphate; MBTPS1/S1P: membrane-bound transcription factor peptidase, site 1; MPR: mannose-6-phosphate receptor; SQSTM1: sequestosome 1; TEM: transmission electron microscopy; TGN: trans-Golgi network.