TM4SF5 is a tetraspanin family member that functions as a lysosomal membrane arginine sensor 1. It forms arginine-regulated complexes with mTOR and SLC38A9 on lysosomal membranes, facilitating arginine efflux and subsequent mTORC1 activation with downstream phosphorylation of RPS6KB1 and EIF4EBP1 1. TM4SF5 promotes G1/S phase cell cycle progression through CDK4-CCND1 nuclear translocation, involving CDKN1B and RHOA/ROCK signaling 2. As a four-transmembrane glycoprotein undergoing N-glycosylation and palmitoylation 3, TM4SF5 assembles tetraspanin-enriched microdomains containing diverse membrane proteins including integrins, growth factor receptors, and amino acid transporters 4. Functionally, TM4SF5 regulates cystine uptake and arginine export 3, induces epithelial-mesenchymal transition via RhoA inactivation 5, and supports macropinocytosis-dependent albumin uptake for hepatocellular carcinoma bioenergetics 6. Clinically, elevated TM4SF5 expression correlates with cancer progression across multiple tissues (esophageal, pancreatic, hepatocellular) and poor patient prognosis 7, 8. TM4SF5 also mediates liver fibrosis development in collaboration with TGFβ1 and EGFR signaling 4, making it a validated therapeutic target for cancer and fibrotic disease.