PIP4P1 (also known as TMEM55B) is a phosphatidylinositol-4,5-bisphosphate 4-phosphatase that catalyzes the hydrolysis of PtdIns(4,5)P2 to PtdIns(5)P 1. The enzyme localizes to late endosomal and lysosomal membranes, where it regulates multiple aspects of lysosomal biology. PIP4P1 recruits the adaptor protein JIP4 to lysosomes, thereby promoting retrograde transport toward the cell center via dynein-dependent mechanisms 2. The protein also contributes to assembly of the V-ATPase complex within lysosomal lipid rafts, facilitating amino acid-dependent activation of mTORC1 signaling 3. Additionally, PIP4P1 modulates low-density lipoprotein receptor (LDLR) trafficking and lysosomal degradation, influencing plasma cholesterol levels 4. Recent evidence suggests that PIP4P1 positively regulates basal autophagy 5. The protein's expression is transcriptionally upregulated following TFEB and TFE3 activation in response to cellular stress 2. PIP4P1 dysfunction has been associated with multiple diseases including Alzheimer disease, intellectual disability, and obesity, though the specific pathogenic mechanisms in these contexts remain to be fully characterized.